Ebook Lippincott illustrated reviews flash cards biochemistry: Part 2

(BQ) Part 2 book Lippincott illustrated reviews flash cards biochemistry presents the following contents: Nitrogen metabolism, metabolism integration, genetic information storage and expression, blood clotting.

18.6 Answer Steroid Hormones

Pregnenolone , derived from cholesterol, is the parent of all steroid

hormones Desmolase, a CYP enzyme (CYP11A, or P450 scc) of the inner

mitochondrial membrane catalyzes cholesterol hydroxylation and

side-chain cleavage (scc) This is the initial and rate-limiting step

ACTH (or corticotropin ) from the pituitary gland stimulates cortisol

synthesis and release Cortisol binds to soluble receptors, and the

receptor–hormone complex binds (as a dimer) to HREs on DNA, thereby

altering gene expression Changes in expression result in increased

gluconeogenesis as well as weakened immune and infl ammatory

responses

Decreased aldosterone and cortisol production and increased

androstenedione production are characteristic of 21- ␣ hydroxylase defi

-ciency , the most common cause of CAH In the classic form, salt wasting

characterized by dehydration, hypotension, hyponatremia, and

hyperka-lemia is seen because of aldosterone defi ciency With 21- ␣ -hydroxylase

defi ciency, masculinization of female genitalia is seen because of androgen

overproduction In contrast, 17- ␣ -hydroxylase defi ciency causes

female-like genitalia in both sexes because of the absence of androgens

Desmolase

(CYP11A ,

P 4 5 0 s c c

3-β-Hydroxysteroid dehydrogenase

11-β-Hydroxylase

(CYP11B1)

17- β-Hydroxysteroid dehydrogenase

Cortisol (21C) Estradiol (18C) Aldosterone (21C)

18- α-hydroxylase

(aldosterone synthase) (CYP11B2)

11- β-hydroxylase)

(CYP11B1)

Overall Nitrogen Metabolism 19.1 Question What are the three inputs to the amino acid pool shown?

What is “protein turnover”?

Compare and contrast the proteasomal and lysosomal systems of protein degradation

What does it mean for an individual to be in N balance? Positive N balance?

Amino acid pool

?

19.1 Answer Overall Nitrogen Metabolism

Three inputs to the amino acid pool are the (1) degradation of body protein,

(2) degradation of dietary protein, and (3) synthesis of nonessential amino acids

“ Protein turnover ” is the ongoing synthesis and degradation of a protein In

a healthy adult, the rate of synthesis is just suffi cient to replace the amount of

protein that was degraded, resulting in a steady state Turnover rate varies

among proteins

Proteasomal protein degradation involves the three-step, ATP-dependent

enzy-matic tagging of proteins with ⱖ 4 Ub followed by cleavage to small peptides

in the cytosolic proteasome as Ub is recycled The proteasomal system is

selective and is infl uenced by structural aspects of the protein In contrast, the

relatively nonselective lysosomal system is ATP and Ub independent and uses

acid hydrolases to cleave proteins

N balance means that the amount of N going into the body equals the amount

going out In a state of positive N balance , however, more N is going in

than is coming out, such as in periods of growth (including pregnancy) and in

recovery from muscle atrophy (e.g., as occurs with prolonged immobilization or

Body protein

Amino acid pool

~400 g/day

Dietary protein can vary from none (for example, fasting) to over 600 g/day (high-protein diets), but

100 g/day is typical of the U.S diet.

Synthesis of nonessential amino acids

19.2 Question

Dietary Protein Digestion

What enzyme of protein digestion, denoted by the red question mark, is produced by the

stomach?

What role does enteropeptidase play in digestion?

Why is celiac disease a pathology of malabsorption?

What amino acids are expected to be present in the urine of an individual with cystinuria ?

peptidases Di- and tri- peptidases

Amino-Amino acids

Trypsin Chymotrypsin Elastase Carboxy- peptidase

Oligopeptides and amino acids

?

Polypeptides and amino acids

SMALL INTESTINE

Dietary protein

TO LIVER

STOMACH

PANCREAS

Dietary Protein Digestion

19.2 Answer

Pepsin , an acid-stable endopeptidase , is secreted by gastric chief cells as the zymogen

pepsinogen [ Note: In the presence of HCL from gastric parietal cells, pepsinogen undergoes

autocatalytic cleavage to pepsin.]

Enteropeptidase , a serine protease of the brush border membrane of intestinal mucosal

cells, cleaves trypsinogen to trypsin , a serine protease that converts all other pancreatic

zymogens to their active forms through cleavage at the carboxyl side of Arg and Lys residues in

the proteins

Celiac disease ( gluten enteropathy ) is a chronic disease of the gastrointestinal tract

caused by an immune-mediated response to gluten (a protein in wheat, barley, and rye) that

atrophies the brush border, resulting in malabsorption

Cystinuria is an AR defect in the transporter that takes up cystine and the dibasic amino acids

ornithine, Arg, and Lys (sometimes represented as COAL ) in the proximal tubules, causing them

to appear in the urine Cystine can precipitate at the acidic pH of urine and form stones in the

urinary tract ( cystine urolithiasis )

peptidases Di- and tri- peptidases

Amino-Amino acids

Trypsin Chymotrypsin Elastase Carboxy- peptidase

Oligopeptides and amino acids

Pepsin

Polypeptides and amino acids

SMALL INTESTINE

Dietary protein

TO LIVER

STOMACH

PANCREAS

19.3 Question

Nitrogen Removal

What is the general name of the enzymes that catalyze the reversible transfer of

amino groups from one carbon skeleton to another, as shown? What vitamin is the

source of the coenzyme used in the reaction?

What is the primary fate of Glu during periods of amino acid catabolism?

Which set of clinical fi ndings in blood is more suggestive of liver disease ?

A ↑AST, ↑ALT, ↑bilirubin

B ↑AST, ↔ALT, ↔bilirubin

CH2

COO–

R C

19.3 Answer Nitrogen Removal

Aminotransferases ( transaminases ) catalyze the reversible transfer of amino groups from most

amino acids to ␣-KG, a process known as transamination The products are an ␣-keto acid and Glu

[ Note: Lys and Thr are not substrates for aminotransferases ] The PLP coenzyme required by these

enzymes is derived from vitamin B 6 ( pyridoxine )

During amino acid catabolism, Glu is oxidatively deaminated to ␣ -KG ⫹ NH 3 by the mitochondrial

enzyme GDH that uses NAD ⫹ as a coenzyme as shown ADP (a low-energy signal) is an allosteric

activator [ Note: The GDH reaction is reversible and the reductive biosynthesis of Glu uses NADPH.]

Choice A (↑ AST , ↑ ALT , ↑bilirubin) is more suggestive of liver disease AST and ALT are

intracel-lular enzymes that leak into the blood when liver cells are damaged The rise in bilirubin indicates a

problem with hepatic metabolism ALT is found primarily in liver, whereas AST is also found in heart

and skeletal muscle and RBCs Therefore, a rise in AST with a normal value for ALT and bilirubin

sug-gests damage to nonhepatic tissues

R

HC NH 3+COO – α-Amino acid

CH

CH2COO–

R C COO –

CH 2

COO–CH

CH 2

COO–

19.4 Question

Ammonia and the Urea Cycle

What is the amino acid product of the reaction shown? Would you expect the enzyme that catalyzes

the reaction to be a synthase or a synthetase ? What is the biologic signifi cance of the reaction?

What is the function of the UC, and where does it occur? What is the regulated enzyme? What is the

fate of the urea product?

How do the liver and the kidneys metabolize Arg differently? How does this relate to Arg being

HCNH3+

ADP + PiATP + NH3

19.4 Answer Ammonia and the Urea Cycle

Gln is the amino acid product Because the catalyzing enzyme requires ATP, it is

a synthetase ( glutamine synthetase ) The reaction utilizes toxic NH 3 (generated

in amino acid catabolism) to form Gln, a nontoxic transporter of NH 3 through the

blood Gln, primarily generated by skeletal muscle, is taken up and metabolized by

the liver, intestine, and kidneys

The UC converts toxic NH 3 to nontoxic urea This ATP-dependent process

occurs in hepatocytes (the fi rst two reactions in the mitochondrial matrix, the

remaining three in the cytosol) [ Note: Gluconeogenesis and heme synthesis

also require enzymes of the matrix and the cytosol.] The regulated enzyme of the

UC is CPS I , which requires N-AcGlu as an allosteric activator Urea, the most

important means of disposing of NH 3 , is transported through the blood to the

kidneys for excretion [ Note: The UC uses and regenerates ornithine.]

The liver expresses the full complement of UC enzymes, including arginase-1

that hydrolyzes Arg to urea and ornithine, whereas the kidney is able to make Arg

from citrulline but does not contain arginase-1 [ Note: Arg is used for renal NO

HCNH3+

ADP + Pi

CO NH2

19.5 Question

Ammonia and the Urea Cycle

What are the sources of the N that appears in urea?

What happens to the fumarate produced by argininosuccinate lyase ?

A 9-month-old boy was admitted to the hospital for evaluation of chronic vomiting and developmental

delay Lab studies revealed elevated levels of NH 3 , Gln, Ala, and ornithine Citrulline was low

Which UC enzyme is defi cient in the patient?

Why might antibiotics be used to treat UC disorders ?

Argininosuccinate

Arginine

Ornithine Carbamoyl phosphate Citrulline

UREA CYCLE

Urea

Fumarate

Oxidative deamination

Ammonia and the Urea Cycle

19.5 Answer

NH 3 , primarily from amino acid catabolism, provides one N of urea,

and Asp provides the other [ Note: Glu is the immediate precursor

of the NH 3 (via GDH ) and of the Asp (via AST ).]

The fumarate produced by cytosolic argininosuccinate

lyase is hydrated to malate, which can be transported into the

mitochondrial matrix, enter the TCA cycle, and be oxidized to OAA

The OAA can be used in gluconeogenesis or transaminated to Asp

and used in the UC

Mitochondrial X-linked OTC is the defi cient enzyme Its defi ciency

is the most common UC disorder, resulting in elevated levels of

NH3 and ornithine (substrates for the cycle and the OTC reaction,

respectively) The rise in NH3 causes a rise in Gln Ala, which

transports N from amino acid catabolism, also increases Citrulline

(the product of OTC) decreases

Because NH 3 is generated from urea by urease of intestinal

bacteria, antibiotic use decreases this source of NH 3 in patients with

UREA CYCLE

Urea

Fumarate

Oxidative deamination

MITOCHONDRIAL MATRIX

P

O–

O O – O

HCO3

+

2 ATP

NH3+

NH3+

CH2CH2 CH2 HCNH3+COO–

L -Ornithine

C

NH2 O NH CH2 CH2 CH2 HCNH3+COO–

Carbamoyl phosphate synthetase I Ornithine trans-

carbamoylase O

19.6 Question

Ammonia Metabolism

How do the UC, glutaminase , and glutamine synthetase shown work together within hepatocytes to keep blood

NH 3 levels low?

What is the signifi cance of NH 4 ⫹ production by the kidney?

What is blood BUN? UUN?

What role does phenylacetate play in UC disorder treatment?

ADP + Pi

METABOLISM

URINE

Amide nitrogen donated in biosynthetic reactions

DIET

α-Ketoglutarate

Glutamate dehydrogenase

H +

Carbamoyl phosphate

19.6 Answer Ammonia Metabolism

Periportal hepatocytes are rich in glutaminase , which produces toxic NH 3 (and Glu) from Gln (its nontoxic carrier),

and in the enzymes of the UC that converts the NH 3 to nontoxic urea for transport to the kidneys Any NH 3 missed by these

reactions is “scavenged” by glutamine synthetase in the perivenous hepatocytes and used to convert Glu to Gln

that is sent out into the blood Together, these processes prevent hyperammonemia , a condition that has a neurotoxic

effect

NH 4 ⴙ production (NH 3 ⫹ H ⫹ → NH 4 ⫹ ) by the kidney helps to maintain acid–base balance through urinary excretion of

H ⫹ , which is important when the rate of ketogenesis is faster than the rate of ketolysis, for example [ Note: Loss of HCO 3 ⫺

in metabolic acidosis decreases the UC Consequently, NH4 ⫹ production increases.]

BUN is a measure of the urea content in blood at a given point in time

UUN is a measure of the urea content in urine over 24 hours

Phenylacetate (from the prodrug phenylbutyrate) conjugates with Gln (a nonessential amino acid) and is excreted in the

urine, thereby decreasing the NH 3 load of the body Such treatment has been shown to reduce the morbidity and mortality

ADP + Pi

METABOLISM

URINE

Amide nitrogen donated in biosynthetic reactions

DIET

α-Ketoglutarate

Glutamate dehydrogenase

H +

Carbamoyl phosphate synthetase I

20.1 Question

Carbon Skeleton Catabolism

Which vertical column (A, B, or C) shown would most appropriately be labeled

“(Solely) Ketogenic Amino Acids”?

Which horizontal row (1 or 2) would most appropriately be labeled “Essential

Amino Acids”? What does it mean for an amino acid to be essential?

The pathways for catabolism of the C-skeletons of amino acids converge to form

what seven intermediate products?

Asparaginase is used to treat childhood acute lymphoblastic leukemia ( ALL )

What is the biochemical basis of this treatment?

Alanine Arginine Asparagine Aspartate Cysteine Glutamate Glutamine Glycine Proline Serine

Histidine Methionine Threonine

Lysine

Isoleucine Phenyl- alanine Tryptophan

20.1 Answer Carbon Skeleton Catabolism

Column C contains the two solely ketogenic amino acids

Row 2 contains the nine essential amino acids, which are the amino acids that

cannot be synthesized (or synthesized in suffi cient quantities) by humans

Catabolism of the C-skeletons of amino acids produces TCA intermediates

( ␣ -KG, succinyl CoA, fumarate, and OAA) and pyruvate from the glucogenic

amino acids, and acetoacetate (or its derivative acetyl CoA) from the ketogenic

amino acids

Asparaginase (from bacteria) is a treatment for ALL because it deamidates

circulating Asn to Asp Rapidly dividing leukemia cells require Asn for growth

and have limited capacity to synthesize it

Alanine Arginine Asparagine Aspartate Cysteine Glutamate Glutamine Glycine Proline Serine

Tyrosine

Histidine Methionine Threonine Valine

Leucine Lysine

Glucogenic and Ketogenic

Isoleucine Phenyl- alanine Tryptophan

Carbon Skeleton Catabolism 20.2 Question

Why might FIGlu, an intermediate in the catabolism of His, as shown, be found in the urine of individuals defi cient in folic acid (folate)?

What is the function of THF?

How is folate defi ciency manifested clinically?

Glutamate

glutamate (FIGlu)

N-Formimino-N

CH2NH

NH

CHN

NH4

20.2 Answer Carbon Skeleton Catabolism

FIGlu reacts with THF to form N 5 -formimino-THF ⫹ Glu If folate (and, consequently, THF) is defi cient, FIGlu accumulates and is excreted in the urine

[ Note: The FIGlu excretion test has been used in the diagnosis of folate defi ciency.]

THF is a carrier of one-C groups (attached to N 5

, N 10

, or both N 5

and N 10

of the molecule) in oxidation states that range from formyl to methyl

[ Note: N 10 -formyl-THF is used in synthesis of the purine ring; N 5 ,N 10 -methylene-THF is used in the synthesis of dTMP from dUMP; and N 5 -methyl-THF is used in the remethylation of Hcy to Met, a reaction that also requires vitamin B 12 This remethylation reaction is the only time THF carries and donates a methyl

group.] THF is made from folate in a two-step, NADPH-requiring reaction catalyzed by DHFR

Folate defi ciency presents as a megaloblastic anemia (a type of macrocytic anemia ) in which cell growth occurs without cell division because of

decreased availability of the purines and the dTMP needed for DNA synthesis [ Note: Vitamin B 12 defi ciency presents in a similar manner.]

Glutamate

N 5 tetrahydrofolate

-Formimino- folate N-Formimino-

Tetrahydro-glutamate (FIGlu)

N

CH2NH

NH

CHN

NH4

20.3 Question

Sulfur-Containing Amino Acids

What is the function of SAM, produced by the metabolism of Met, as shown?

Cys, produced from the sulfur of Met and the C-skeleton of Ser (shown), can be desulfurized to pyruvate What is an

important use of the sulfate released in this process?

Why is homocystinuria a concern? What role do vitamins B 6 , B 12 , and folate play in maintaining low Hcy levels?

L -Homocysteine

CH2COO –

HCNH3+

CH2SH

H 2 O S-Adenosylhomocysteine (SAH)

HCNH3+

CH2Adenosine

Adenosine

CH 3

P i + PP i

2 P i ATP

H 2 O L-Serine

Methionine adenosyl- transferase

Mg 2 +

L-Methionine

Adenosine

SAH hydrolase

20.3 Answer Sulfur-Containing Amino Acids

SAM (like THF) is a one-C carrier, but (unlike THF) SAM carries only methyl groups , which are transferred by

methyltransferases to acceptors such as norepinephrine, PE, DNA, and RNA

The sulfate released from the desulfurization of Cys can be used to synthesize PAPS , an activated sulfate donor

with a variety of acceptors (e.g., the GAGs )

Homocystinuria , caused by elevated Hcy levels, promotes endothelial dysfunction and is an independent risk

factor for occlusive vascular disease Hcy is kept low by (1) conversion to Cys, a two-step, B 6 -dependent

process (shown) catalyzed by cystathionine synthase and cystathionase , and (2) remethylation to Met,

a THF- and B 12 -requiring reaction catalyzed by methionine synthase As the levels of vitamins B 6 , B 12 ,

and folate ↓, Hcy levels ↑ Mild elevations of Hcy are seen in a small percentage of individuals, but large

elevations are rare and are primarily seen in cystathionine ␤ -synthase defi ciency

Methyl acceptors

Methylated products

Mg2 +

L -Methionine

Adenosine

B6 B6

SAH hydrolase

N 5 tetrahydrofolate Homocysteine

cobalamin)

(methyl-Methionine synthase

20.4 Question

Branched-Chain Amino Acids

What coenzymes are required by BCKD , the enzyme that oxidatively decarboxylates the ␣ -keto acid derivatives of the

BCAAs, as shown? What other enzymes also require them?

In addition to Val and Ile, what other amino acids are metabolized to propionyl CoA and, ultimately, succinyl CoA?

Why are individuals with maple syrup urine disease ( MSUD ), a rare AR disorder caused by BCKD defi ciency,

at particular risk during periods of physiologic stress?

ACETATE +

ACETO-ACETYL CoA

Propionyl CoA

Isovaleryl CoA Isobutyryl CoA

butyryl CoA

α-Methyl- caproic acid α-Ketoiso- valeric acid α-Keto-β-methyl- valeric acid

α-Ketoiso-Leucine Valine Isoleucine

crotonyl CoA

β-Methyl-Methylmalonyl CoA

Biotin Biotin FAD-linked DEHYDROGENATION

20.4 Answer Branched-Chain Amino Acids

BCKD , a mitochondrial enzyme, requires NAD ⫹ and CoA as cosubstrates and TPP, lipoic acid, and FAD

as prosthetic groups PDH and ␣ -KGD are the other mitochondrial ␣ -keto acid dehydrogenase

complexes that require this group of coenzymes

In addition to Val and Ile, Met and Thr are metabolized to propionyl CoA Biotin-dependent propionyl

CoA carboxylase converts propionyl CoA to methylmalonyl CoA, which is converted to succinyl CoA by

a B 12 -dependent mutase The other B 12 -requiring reaction in humans is the remethylation of Hcy to Met

[ Note: FAs with an odd number of C atoms produce propionyl CoA in the fi nal round of ␤ -oxidation.]

Physiologic stress triggers skeletal muscle proteolysis to meet increased energy needs Because Val

and Ile provide glucose (from the metabolism of succinyl CoA to glucogenic Ala) and Leu and Ile provide

acetyl CoA, these energy sources will be in decreased supply in individuals with MSUD , putting them at

particular risk during periods of physiologic stress Additionally, elevated Leu can cause neurologic

dam-age [ Note: BCKD defi ciency confers a maple syrup–like odor to body fl uids.]

ACETATE + ACETYL CoA

ACETO-ACETYL CoA

Propionyl CoA

Isovaleryl CoA Isobutyryl CoA

butyryl CoA

`-Methyl- caproic acid

valeric acid

`-Ketoiso- valeric acid

`-Keto-a-methyl-Leucine Valine Isoleucine

TRANSAMINATION

(Branched-chain amino acid aminotransferase)

HMG CoA

glutaconyl CoA

crotonyl CoA

a-Methyl-SUCCINYL CoA

Methylmalonyl CoA

Biotin Biotin

5'-Deoxyadenosyl- cobalamin (derivative of B 12 )

20.5 Question

Aromatic Amino Acids

What coenzyme is required by the PAH reaction shown?

What other enzymes of amino acid metabolism require this coenzyme?

What is the cause of phenylketonuria ( PKU ), and how is it treated? Why are the CNS effects of

PKU now rarely seen?

What is the clinical consequence of tyrosinase defi ciency ?

20.5 Answer Aromatic Amino Acids

THB ( BH 4 ), made from GTP, is the coenzyme for the PAH reaction Its defi ciency

results in hyperphenylalaninemia and decreased Tyr production

BH 4 is also used by tyrosine and tryptophan hydroxylases Its defi ciency

decreases synthesis of the catecholamines from Tyr and serotonin from Trp

Treatment includes replacement therapy [ Note: Use of BH 4 by aromatic amino

acid hydroxylases (and by NOS that synthesizes NO from Arg) is in contrast to

the use of PLP in most other reactions involving amino acids.]

PAH defi ciencies cause PKU , which is characterized by a “mousey” odor

Treatment includes Phe restriction and supplementation with the now-essential

Tyr Newborn screening programs have allowed early diagnosis and treatment of

PKU, preventing the microcephaly , intellectual disability , and seizures ,

characteristic of the untreated defi ciency Because Phe is teratogenic , women

with PKU can give birth to children with anatomic anomalies if Phe levels are not

controlled ( maternal PKU syndrome )

Lack of tyrosinase , which is required for the synthesis of melanin from Tyr,

causes oculocutaneous albinism

Defi ciency of homogentisic acid oxidase of Tyr catabolism causes

alkap-tonuria Symptoms include formation of a blue-black pigment-like polymer

( ochronosis ) in connective tissue (and urine) and early-onset arthritis

biopterin + H2O

Dihydro- biopterin + O2

Tetrahydro-Phenylalanine hydroxylase

CH2

NH3+

L-Phenylalanine

C COO–H

PKU

21.1 Question

Heme Structure and Synthesis

Based on the fi gure, to what series of the porphyrins (cyclic tetrapyrroles) does

heme belong? What are some examples of heme-containing proteins?

What are the major sites of heme synthesis in the body? What subcellular sites

are involved? What is the rate-limiting, committed reaction?

Why might use of statins , cholesterol-lowering drugs metabolized by the hepatic

CYP system, cause an increase in heme synthesis in the liver?

Heme

Fe

CH2

CH2–

21.1 Answer Heme Structure and Synthesis

In heme , the side chains are asymmetrically distributed on one of the pyrrole rings , placing

heme in the III series of the porphyrins [ Note: IX refl ects an older naming system and is equivalent to

III.] Hb, Mb, CYP monooxygenases , NOS , and catalase , are examples of proteins that contain heme, a

metalloporphyrin that functions as a prosthetic group

The liver and RBC-producing cells of the marrow are the major sites of heme synthesis, with ⬎ 85%

occurring in the marrow Enzymes of the mitochondria and the cytosol are required The rate-limiting,

committed step is the mitochondrial synthesis of ALA from Gly and succinyl CoA by PLP-requiring

isozymes, ALAS1 and ALAS2

ALAS1 (the ubiquitous isozyme) is regulated by heme (shown), which represses gene

transcrip-tion, increases mRNA degradatranscrip-tion, and decreases enzyme import into mitochondria Use of heme in

the synthesis of the hepatic CYP enzymes needed to metabolize the statins prevents heme from

accumulating This favors activation of ALAS1 and, consequently, heme synthesis in the liver

[ Note: ALAS2 (the isozyme specifi c to erythroid tissue) is regulated by iron: as iron ↑, synthesis of

CH3

CH3 CH2 H2C CH

CH

CH2 CH2 COO –

CO 2

COO–

CH2

CH2O C

21.2 Question

Heme Synthesis

What metal inhibits the reaction shown? Are any other reactions of heme synthesis

similarly affected?

How is protoporphyrin IX synthesized from porphobilinogen?

What are the porphyrias , and how are they classifi ed? Which is most common?

(cytosolic enzyme)

(Two moleculescondense)

δ-Aminolevulinic acid (ALA)

2 H2O

21.2 Answer Heme Synthesis

Lead inhibits the ALA dehydratase - catalyzed condensation of two ALA to porphobilinogen , a

pyrrole Mitochondrial ferrochelatas e , which inserts Fe 2 ⫹ into protoporphyrin IX in the fi nal step

of heme synthesis, is also inhibited Consequently, lead poisoning causes microcytic anemia

Four porphobilinogens are condensed in the cytosol to hydroxymethylbilane (a linear

tetrapyrrole), which is enzymatically cyclized and isomerized to uroporphyrinogen III , which,

in turn, undergoes UROD -catalyzed decarboxylation of all its acetate groups (to methyl) to

produce coproporphyrinogen III This product moves into the mitochondrion and undergoes

decarboxylation and oxidation of two propionyl groups (to vinyl) to form protoporphyrinogen IX ,

which gets oxidized to protoporphyrin IX Insertion of Fe 2 ⫹ yields heme [Note: Defi ciency of

uroporphyrinogen III synthase results in overproduction of the I series porphyrins.]

Porphyrias are rare, inherited (AD, primarily) or acquired (e.g., lead poisoning ) enzymatic defi

-ciencies in heme synthesis in which porphyrins (or their precursors) accumulate and are excreted

They are classifi ed as hepatic or erythropoietic Hepatic forms are further classifi ed as acute or

chronic PCT , a chronic porphyria caused by UROD defi ciency , is the most common Patients

are photosensitive due to the light-induced oxidation of porphyrinogens to porphyrins

Cutaneous symptoms (shown) and urine that turns reddish-brown are seen [Note: In the hepatic

porphyrias ↓ heme synthesis ↑ ALAS1 activity, thereby allowing synthesis of intermediates prior

to the defective enzyme Their accumulation causes the clinical manifestations of the porphyrias.]

δ-Aminolevulinic acid (ALA)

2 H2O

21.3 Question

Heme Degradation

What is the primary source of the heme that is degraded by macrophages of the RES, as

shown? What enzyme catalyzes the initial step of degradation?

What is the function of bilirubin UGT ? What is the fate of its product?

What is the difference between Dubin-Johnson and Crigler-Najjar I syndromes ?

Biliverdin

CO

MACROPHAGE

Biliverdin reductase

O O

V

N H

21.3 Answer Heme Degradation

About 85% of the heme degraded by RES macrophages comes from the Hb of senescent RBCs , and

the rest comes from proteins other than Hb Microsomal heme oxygenase uses O 2 and NADPH to convert

cyclic heme to linear biliverdin CO, Fe 2 ⫹ , and NADP ⫹ are also produced Biliverdin is reduced to bilirubin

by NADPH-requiring biliverdin reductase , enters into the blood, and is bound by albumin for transport to

the liver

Bilirubin UGT is the hepatic microsomal enzyme that converts bilirubin to bilirubin diglucuronide

(thereby increasing its solubility) through the addition of two molecules of glucuronate from UDP–glucuronic

acid Bilirubin diglucuronide ( CB or direct bilirubin ) is secreted into bile Intestinal bacteria hydrolyze and

reduce it to urobilinogen , most of which is oxidized to stercobilin , which colors feces Some, however, is

reabsorbed into blood, taken up by liver, and secreted into bile ( enterohepatic circulation ) The remainder

is transported to the kidneys, converted to yellow urobilin , and excreted [ Note: Because CB is normally sent

into the intestine, ⬎ 95% of the total serum bilirubin is UCB.]

Dubin-Johnson syndrome (benign) is caused by a rare defi ciency in the protein that transports CB out

of the liver, causing it to leak into blood and resulting in a conjugated (direct) hyperbilirubinemia

Crigler-Najjar I (severe) is a virtually complete defi ciency of bilirubin UGT that results in an unconjugated

Bilirubin–albumin complex

Bilirubin

BILE

Bilirubin diglucuronide

2 UDP-glucuronic acid

Bilirubin UDP- glucuronosyl- transferase

21.4 Question

Jaundice

Deposition of what molecule is responsible for the yellow color of the sclerae shown, a condition

known as jaundice ?

What are the three major types of jaundice?

Which type of jaundice is best represented by the fi gure?

Heme Biliverdin, CO, Fe2+

Hemoglobin, Cytochromes Amino acids Erythrocytes, heptocytes

Bilirubin glucuronide Bilirubin

Bilirubin

?

21.4 Answer Jaundice

Jaundice ( icterus ) is caused by deposition of bilirubin secondary to hyperbilirubinemia

The three major types of jaundice are (1) hemolytic ( prehepatic ) jaundice caused by

production of bilirubin in excess of the liver’s capacity to conjugate it, resulting in an

unconju-gated hyperbilirubinemia; (2) hepatocellular ( hepatic ) jaundice caused either by bilirubin UGT

defi ciency, resulting in an unconjugated hyperbilirubinemia, or by impaired secretion of CB into

bile, resulting in a conjugated hyperbilirubinemia; and (3) obstructive ( posthepatic ) jaundice

caused by common bile duct blockage, resulting in a conjugated hyperbilirubinemia

[ Note: If less CB enters the intestine, stool is pale in color The resulting increase in urinary

bilirubin darkens the urine Only CB is found in urine because it is water soluble UCB is not.]

Hepatocellular jaundice , caused by decreased hepatic production or secretion of CB, is

represented [ Note: Physiologic jaundice of the newborn , a type of hepatocellular jaundice

caused by a transient developmental delay in bilirubin UGT expression, is seen in the majority of

neonates If UCB levels exceed the binding capacity of albumin, UCB can cross the BBB and cause

a toxic encephalopathy known as kernicterus Treatment includes phototherapy to convert

bilirubin to a more water-soluble isomer.]

Hepatocellar jaundice

Heme Biliverdin, CO, Fe2+

Hemoglobin, Cytochromes Amino acids Erythrocytes, heptocytes

Bilirubin glucuronide Bilirubin

Urobilinogen Bilirubin

Stercobilin Urobilin

21.5 Question

Catecholamines

What enzyme catalyzes the rate-limiting conversion of Tyr to DOPA, as shown? What coenzyme does it require? What coenzyme is required in the conversion

of DOPA to dopamine? Of norepinephrine to epinephrine?

What is the function of the norepinephrine and epinephrine (catecholamines) released from the adrenals in response to physiologic stress?

Cu 2+

Ascorbate + O2

ascorbate + H2O

Dehydro-Dopamine β-hydroxylase Phenylethanolamine-

transferase

21.5 Answer Catecholamines

Tyrosine hydroxylase converts Tyr to DOPA As an aromatic amino acid hydroxylase , it requires THB as a coenzyme The conversion of DOPA to

dopamine is catalyzed by aromatic amino acid decarboxylase , which requires PLP The methyltransferase that converts norepinephrine to epinephrine

requires SAM [ Note: Only the methylation of Hcy to Met uses THF.]

Catecholamines mediate the retrieval of energy-producing molecules from tissue stores in times of physiologic stress

In Parkinson disease , the loss of dopamine-producing cells in the brain results in dopamine defi ciency Dopamine is degraded to HVA by COMT

and MAO , so a defi ciency results in decreased HVA generation [ Note: COMT and MAO degrade epinephrine and norepinephrine to VMA.]

Tyrosine hydroxylase

transferase

S-Adenosyl- biopterin + O 2

Tetrahydro- biopterin + H 2 O

Dihydro- ascorbate + H 2 O

Dehydro-Ascorbate + O 2

Aromatic amino acid decarboxylase

acetic acid

Dopamine

21.6 Question

Other Nitrogen-Containing Compounds

What is the function of creatine phosphate, the synthesis of which from the amino acids Arg, Gly, and

Met (as SAM) is shown?

Why are SSRIs used to treat depression and anxiety disorders?

Why do the antihistamines used to treat allergies have no effect on histamine-mediated gastric acid

Amidino-Ornithine Guanidinoacetate

ADP

Creatine phosphate

Creatinine

Creatine kinase

P i

21.6 Answer Other Nitrogen-Containing Compounds

Creatine phosphate provides a small, rapidly mobilized muscle

reserve of high-energy phosphate groups that can be transferred to

ADP to maintain ATP levels early in intense contraction It

sponta-neously cyclizes to creatinine , which is fi ltered by the kidney with

little reabsorption Elevated creatinine levels in the blood with low

levels in the urine indicates impaired renal function

Serotonin ( 5-HT ), made from Trp in presynaptic vesicles and then

released into the synapse, causes a feeling of well-being Its actions

are terminated by reuptake SSRIs target the 5-HT transporter and

inhibit reuptake, thereby allowing a return of positive feelings

Histamine , made from the PLP-requiring decarboxylation of His,

binds to four GPCRs, H1–H4 Allergy drugs block H1 receptors but

have no effect on H2 receptors on acid-secreting parietal cells of the

stomach Specifi c H2 blockers are required to reduce acid secretion

Arginine

H 2 0

Glycine

transferase

Amidino-Ornithine Guanidinoacetate

Tetrahydro-PLP Dihydro-

+ H 2 O

22.1 Question

Nucleotide Structure and Function

Which nitrogenous base shown is used (as a component of a nucleotide) in DNA

but not RNA synthesis? In addition to a purine or pyrimidine base, what are the

other two components of a nucleotide?

Why are nucleotides and nucleosides referred to as N-glycosides?

What is the role of nucleotide sugars in the body? What group of disorders

results from defects in nucleotide sugar-dependent protein N-glycosylation ?

N

N

HN

N H N

N H

HN

N H

N

N H HN

22.1 Answer Nucleotide Structure and Function

T (as dTTP ) is used in DNA synthesis, whereas U (as UTP ) is used in RNA synthesis Structurally, T is methylated U In addition to a purine or pyrimidine nitrogenous base, a nucleotide contains a pentose monosaccharide (ribose in RNA and 2-deoxyribose in DNA) plus one to three phosphate groups

[ Note: Compared to a nucleotide, a nucleoside lacks phosphate groups The terms “nucleoside phosphate” and “nucleotide” are used interchangeably.]

Nucleotides and nucleosides are referred to as N-glycosides because a N in the base is linked to C-1' of the sugar [ Note: The number of a C atom in the

sugar includes a prime sign (') to distinguish it from the atoms in the base.]

Nucleotide sugars are activated monosaccharide donors in the synthesis of polysaccharides, glycoproteins, proteoglycans, and glycolipids For example, UDP-glucose is used in glycogen synthesis, GDP-mannose in glycoprotein synthesis, and CMP-NANA in ganglioside (glycolipid) synthesis Congenital

disorders of glycosylation (CDG) result from defective production, transport, and processing of nucleotide sugars required for protein N-glycosylation.

N HN N N

N HN

N N

N HN

NH 2

N N O HO

1 3 6

1' 4' 5'

NH 2

N

N

3 1 4

N

N

9 7

3 ' 2 '

H O

OH

1' 4' 5'

HO

3 ' 2 '

22.2 Question

Purine Nucleotide De Novo Synthesis

The origins of the atoms in a purine base during nucleotide de novo synthesis

are shown What is the order of addition of these atoms?

What enzyme catalyzes the committed step of purine nucleotide de novo

synthesis How is it regulated? What is the fate of IMP, the fi rst purine

nucleotide made?

Why does methotrexate cause a decrease in DNA synthesis? Why do

sulfonamides decrease DNA synthesis in bacterial but not human cells?

N N

N C

C C C

Glutamine

N

C

CO2Aspartate

N10 tetrahydrofolate

-Formyl-Glycine

22.2 Answer Purine Nucleotide De Novo Synthesis

The order of the addition of atoms in purine base synthesis is (1) the amide N from Gln, (2) the N ⫹ C atoms from Gly, (3) a C from N 10 -formyl-THF, (4) the amide N from another Gln, (5) the C from CO 2 , (6) the N of Asp, and (7) a C from another N 10 -formyl-THF

The committed step of purine nucleotide de novo synthesis is catalyzed by glutamine:PRPP amidotransferase (shown) The enzyme is activated by PRPP

and inhibited by AMP and GMP IMP , the fi rst purine nucleotide made, is converted to AMP and GMP in separate two-step, energy-requiring processes

Methotrexate inhibits DHFR , which catalyzes the reduction of DHF to the THF required (as N 10 -formyl-THF ) for purine synthesis Sulfonamides inhibit

folate (and consequently THF) synthesis in bacteria With each drug, ↓ purines cause ↓ DNA synthesis Humans, however, cannot synthesize folate and are unaffected by sulfonamides

N N

N C C C C

Glutamine

N C

CO 2 Aspartate

N 10 tetrahydrofolate

-Formyl-Glycine

1 2 4 5 6 7

pyrophosphate amidotransferase

Glutamine:phosphoribosyl-Glutamate + PP i

22.3 Question

Purine Nucleotide Degradation

What highly oxidized purine (indicated by a red question mark) is the end product of purine nucleotide

degrada-tion? How is it excreted from the body? What enzyme catalyzes its production from xanthine? Where is this

enzyme found primarily?

AMP can be deaminated to IMP by AMP deaminase and then converted to inosine by a 5 '- nucleotidase By what

other path is AMP converted to inosine? What pathology results from an enzymatic defi ciency in this conversion?

Why might administration of recombinant uricase be a rational approach to gout treatment?