SLIDE BÀI GIẢNG HÓA SINH vitamin

• first vitamin discovered was thiamine or B1 • the term vitamin is derived from the fact that the substances are needed for life vita and because thiamine happened to be an amine th

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Reading material

• Principles of Biochemistry with a Human

Focus by Garrett and Grisham, First

Edition, 2002, pages 453-468

• Handbook of NonPrescriptions Drugs,

11th edition, Chapter entitled “Nutritional Products” by Loyd V Allen, Jr.

• a group of organic compounds needed in small

quantities in the diet for normal activity of

tissues

• between 14 – 20 substances have been identified

as vitamins

• many vitamins act as cofactors, coenzymes or

prosthetic groups for enzymes

• most vitamins are derived from diet

• no calories are derived from vitamins

• first vitamin discovered was thiamine or

B1

• the term vitamin is derived from the fact

that the substances are needed for life

(vita) and because thiamine happened to

be an amine the term was coined as such

• however, not all vitamins are amines or

nitrogen containing compounds

• vitamin requirements are usually

expressed as RDA’s (recommended

dietary allowances)

• guidelines are provided by 2

organizations:

• the Food and Nutrition Board of the National

Academy of Sciences- National Research Council

• the Food and Drug Administration (FDA)

• applications of RDAs include:

• evaluating the adequacy of the national food

supply

• establishing standards for menu planning

• establishing nutritional policy for public

institutions/organizations and hospitals

• evaluating diets in food consumption studies

• establishing labeling regulations

• setting guidelines for food product formulation

• developing materials for nutritional education

• RDAs have limitations:

• they are too complex for direct consumer use

• they do not state ideal or optimal levels of intake

• the allowances for some categories are based on

limited data

• the data on some nutrients in foods is limited

• they do not evaluate nutritional status

• they do not apply to seriously ill or malnourished

patients

Vitamin deficiencies

• primary food deficiency

• crop failure

• food storage loss

• food preparation loss

• diminished food intake

• poverty

• anorexia

• food fadism

• chronic diseases

Vitamin loss

Loss is seen mainly in storage or food preparation

• Vitamin A: sensitive to oxygen and light

• Vitamin D: usually little loss

• Vitamin E: sensitive to oxidation especially

when heated or with alkali

• Vitamin K: sensitive to acids, alkali, light and

oxidizing agents

• Vitamin C: very sensitive to oxidation,

especially when heated in contact with metals

• Vitamin B complex: water solubility results in

loss in cooking water

• Riboflavin is sensitive to light

• The B vitamins (B 1 , B 2 , B 3 , B 6 , B 7 , B 12 and pantothenic acid)

• Ascorbic acid (vitamin C)

Bogus vitamins

• provide “chemical teeth” for enzymes

• sometimes referred to as coenzymes

• enzymes: proteins with catalytic activity

– simple enzymes: large protein (polypeptide) that

catalyzes a reaction The enzyme gets all the “tools” (chemical teeth) it needs from the amino acids

However, there are only 20 different amino acids

– conjugated enzymes : apoenzyme + cofactor =

holoenzyme

EXAMPLE:Proteases: enzymes that cleave

peptide bonds

N

N

N H

O

H

H 2 O protease

Enzymes perform catalytic reactions such as hydrolysis; the side chains of amino acids participate in the reactions

CH2

N

H N

CH2-COOH

all these tools come from amino acids

in the protein active site

Usually electron-rich side chains are involved

in the catalysis

Aliphatic chains are normally involved in hydrophobic interactions

example of a simple enzyme

A serine protease enzyme such as chymotrypsin

Example of a conjugated enzyme

N

N R

O

H

H

N R'

O

Zn+2OH

cofactor needed for reaction

PRODUCTS + ENZYME

Zinc protease such

as ACE

• all water-soluble vitamins with the exception of

vitamin C are converted/activated to cofactors

• only vitamin K of the fat-soluble vitamins is

converted to a cofactor

• not all vitamins are cofactors; i.e., lipoic acid is

not a vitamin

• cofactors may also act as carriers of specific

functional groups such as methyl groups and acyl groups

The water soluble

vitamins

Pantothenic acid (vitamin B5)

Pantothenic acid

• a yellow viscous oil (free acid)

• stable to moist heat (not to dry heat) and

to oxidizing and reducing agents

• hydrolyzed in acid or alkaline medium

• sources (numerous): liver, kidney, eggs,

lean beef, milk, molasses, cabbage,

cauliflower, broccoli, peanuts, sweet

potatoes, kale (derive its name from

everywhere)

Pantothenic acid

• serves in its activated form as the cofactor for

coenzyme A (CoA) and the acyl carrier protein (ACP)

• first phosphorylated by ATP to

4’-phosphopantothenate

• next is the formation of 4’-phosphopantetheine by

addition of cysteine and decarboxylation

• adenylation by ATP forms dephospho-CoA

• phosphorylation to the 3’-OH of the ribose generates

CoA (coenzyme A)

H H

Acetyl CoA

Coenzyme A

• performs a vital role by transporting acetyl

groups from one substrate to another

• the key to this action is the reactive thioester

bond in the acetyl form of CoA

• the thioester bond is stable enough that it can

survive inside the cell, but unstable enough that acetyl-CoA can readily transfer the acetyl

group to another molecule

Example of an acetylation reaction

Acetylcholine is an important neurotransmitter in

the autonomic nervous system (cholinergic) and in the brain

• inflammation of nasal mucosa

• hemorrhage of adrenal cortex – humans

• has not been encountered or extremely rare

• difficult to induce with either synthetic diets

and/or with antagonists methylpantothenic acid

(omega-Pantothenic acid

• vague symptoms in human deficiency:

• numbness and tingling in feet “burning foot”

• fatigue

• GIT disturbances

• available pharmaceutically as calcium

pantothenate (d-isomer) and as racemic

Vitamin B 1 ; antiberi-beri vitamin; antineuritic factor

was the first water soluble vitamin discovered (Eijkman)

• has the odor and flavor of yeast

• slowly destroyed by moist heat; more

rapidly destroyed in a basic medium than

in an acid one

• source: whole cereals and grains; yeast;

organ meat

• pharmaceutical products use the

hydrochloride or mononitrate salts

• active form is thiamine pyrophosphate (formed

by the action of thiamine

diphosphotransferase)

• involved in the oxidative decarboxylation of

pyruvic acid and α-ketoglutaric acid

• involved in the transketolase reactions of the

triose phosphate pathway

• also required for nerve function (unrelated to

coenzyme activity)

Conversion of thiamine to TPP

Typical reactions catalyzed by

TPP

Reactions in which thiamine pyrophosphate is a cofactor

Thiamine pyrophosphate

• the key portion of this cofactor is the

thiazolium ring with its acidic hydrogen

• the hydrogen is removed by the enzyme

forming an ylid (anion next to cation)

• the anion can then react with carbonyl groups

in such molecules as pyruvate

• the pyrophosphate functionality acts as a

chemical handle which holds the cofactor in place within the enzyme

N S

CH3H

Cl

N S

pyruvate

N S

CH3

Cl

HO

O

O-N S

H OH C

H OH

CH 2 -OPO 3 H 2

C

O H

OH H

D-xylulose-5-phosphate D-ribose-5-phosphate

C

C OH C

H OH

CH 2 -OPO 3 H 2

H OH C

H

H HO

C

CH 2 OH O

C

C OH H

O H

CH 2 -OPO 3 H 2 +

septulose-7-phosphate

3-phosphoglyceraldehy

transketolase TPP

Transketolase reaction

O H

CH 2 -OPO 3 H 2 +

OH H

CH 2 -OPO 3 H 2

C

H OH

C H HO

C

CH 2 OH O

C

H OH

CH 2 -OPO 3 H 2 D-fructose-6-phosphate

These reactions provide a link between the pentose phosphate pathway and

glycolysis

Activity of erythrocyte transketolase is commonly used as an index of

thiamine deficiency

Thiamine deficiency (severe)

• beri-beri (once associated with white

polished rice diets and with highly milled wheat diets)

• 2 clinical types

• dry beri beri or neuritic beriberi

– associated with polyneuropathy (depressed peripheral

nerve function, sensory disturbance, loss of reflexes and motor control and muscle wasting

• wet beri beri or cardiovacular beriberi

– edema, congestive heart failure

N

N

S OH

H 3 C H 3 C

CH 2 -CH 2 -OH OXYTHIAMINE

These 2 compounds are potent antithiamine agents which may

be used to induce symptoms of vitamin B 1 deficiency in selected

animals Oxythiamine competitively inhibits thiamine pyrophosphate and becomes active after phosphorylation; neopyrithiamine

prevents the conversion of thiamine to thiamine pyrophosphate

Other clinical applications

• Alcohol neuritis (peripheral neuropathy)

• Sharp burning pain in the feet

• Deep muscle tenderness with numbness

• Coarse tremors, foot drop

• Wernicke’s encephalopathy

• Results from degeneration of basal ganglia due to

chronic/heavy use of alcohol

Other clinical applications

• Korsakoff’s syndrome or psychosis

• Also a complication of chronic/heavy use of alcohol

• Usually follows DT’s (delirium tremens)

Requirement for thiamine

• Based on energy needs

– 0.3 – 0.6 mg/1000 calories

– Increased requirements:

• Pregnancy and lactation

• Eating large amounts of raw sea food (clams) –

Thiamine assay

and costly (curative or protective)

• microbiologic using bacteria which require

thiamine for growth

• chemical/fluorescent assay – conversion of

thiamine to thiochrome by alkaline ferricyanide

Lipoic acid

• lipoic acid is a co-factor found in pyruvate

dehydrogenase and α-ketoglutarate

dehydrogenase, two multienzymes involved in

α-keto acid oxidation

• lipoic acid functions to couple acyl group

transfer and electron transfer during oxidation and decarboxylation of α-ketoacids

• no evidence exists of a dietary lipoic acid

requirement in humans; therefore it is not

considered a vitamin

H

CH NH

lipoic acid, oxidized form lipoic acid, reduced form

lipoamide complex (lipoyl-lysine conjugate)

Lipoic acid exists in 2 forms: a closed-ring disulfide form and

an open-chain reduced form; oxidation-reduction cycles interconvert these 2 species; lipoic acid exists covalently attached in an amide

linkage with lysine residues on enzymes

• vitamin B 2 , lactoflavin (ovo, hepato, verdo),

vitamin G

• a heterocyclic flavin linked to ribose analogous

to the nucleosides in RNA

• orange-yellow fluorescent compound

• found in significant quantities in green leafy

vegetables, milk and meats

• heat stable, but easily destroyed by light

• recommended intake is related to energy intake

(kcal) – RDA 1 – 2 mg/day

LUMIFLAVIN (produced by photochemical cleavage

of riboflavin under alkaline conditions)

alloxan

4-amino-1,2-dimethyl 5-methylaminobenzene

Decomposition of riboflavin

• 2 cofactors are involved:

– riboflavin phosphate (flavin mononucleotide,

FMN)

– flavin adenine dinucleotide (FAD)

• involved in the metabolism of

carbohydrates, fats and proteins (flavin

dehydrogenases/flavoproteins)

• hydrogen carriers in the respiratory

chain

NH 2

OH OH

H H

FLAVINE ADENINE DINUCLEOTIDE

Riboflavin

• In some enzymes, the cofactor is covalently

bonded to an amino acid (dehydrogenases)

Amino acid oxidases

C

R

NH 3 + H

Xanthine oxidase

N

N N

N OH

H

N

N N

N OH

H HO

N

N N

N OH

H HO

Fatty acyl-CoA desaturase

FAD FADH 2 fatty acyl-CoA desaturase

Important step in the biosynthesis of unsaturated fats; this reaction is actually more complex than shown here and

involves other cofactors, but FAD is a key cofactor for the enzyme

Riboflavin deficiency

• seldom seen in industrialized societies

• deficiency when seen:

• cheilosis (vertical fissure in the lips)

• angular stomatitis (craks in the corner of the mouth)

• glossitis

• photophobia

• seborrheic dermatitis

• normochromic normocytic anemia

• usually encountered along with pellagra (niacin deficiency)

• newborns treated for hyperbilirubinemia by phototherapy

(riboflavin is unstable to light)

N N

S

O

H H

(CH 2 ) 4 -COOH

BIOTIN

• an imidazole sulfur containing compound

• sometimes referred to as vitamin B7 or vitamin H

• widely distributed in foods (liver, kidney, milk,

molasses)

• a large portion of the daily need of biotin is met by

synthesis by intestinal bacteria

• deficiency is usually the result of a defect in

utilization rather than simple dietary deficiency

• like lipoic acid, biotin is converted to its

coenzyme form (called biotinyllysine or biocytin) by formation of a covalent

amide bond to the nitrogen of a lysine residue

• like lipoic acid it performs a highly

specialized function : adds a carboxyl group to substrates

• biochemical role: carbon dioxide

fixation

• two step process:

1 Binding of CO 2 to biotin – N-carboxybiotin

2 Transfer of CO 2 to a substrate

– Activation of biotin requires enzyme,

CO2, ATP and Mg++

Biotin-dependent enzymes:

• Pyruvate carboxylase (synthesis of oxaloacetate

for gluconeogenesis and replenishment of the citric acid cycle)

• Acetyl CoA carboxylase (fatty acid biosynthesis)

• Propionyl-CoA carboxylase β-methylcrotonyl-CoA carboxylase

• holocarboxylase synthase (multiple carboxylase)

Reactions involving biotin enzymes

O

CO 2 -O 2 C

-CH 3 methylmalonyl CoA

HCO 3 - + NH 4 + + ATP H 2 N C

O

O P OH

OH O

carbamyl phosphate

• deficiency:

• quite uncommon

• can be induced by feeding raw egg white (avidin)

• avidin is a protein which binds tighly with biotin (MW

70,000)

• symptoms are: anorexia, nausea, muscle pain, fine scaly

desquamation of the skin

• requirements: 150 – 200 mcg/day

• therapeutic use: in babies with infantile

seborrhea (cradle cap) and Leiner’s disease

Pyridoxine (vitamin B6)

N

CH 2 OH

CH 2 OH HO

H 3 C

PYRIDOXINE

A pyridine derivative

CHO

CH 2 OH HO

CH 2 NH 2

CH 2 OH HO

H 3 C

PYRIDOXAMINE PYRIXOXAL

Other forms of B-6

Collectively, pyridoxine, pyridoxal and pyridoxamine are known as vitamin B 6

• vitamin B6, rat “acrodynia factor”,

antidermatitis factor

• widespread occurrence

• pyridoxine: mostly in vegetable products

• pyridoxal and pyridoxamine: mostly in animal

products

• pyridoxine is stable in acid solution, but

unstable in neutral or alkaline solutions (destroyed by light)

pyridoxal phosphate pyridoxamine phosphate

N

HO

H 3 C

CH 2 OH COOH

pyridoxic acid

Pyridoxal phosphate

• pyridoxine is converted to pyridoxal phophate

by phosphorylation and oxidation to the

aldehyde

• pyridoxal phosphate is then attached to the

holoenzyme via a covalent bond to a lysine

residue (a Schiff’s base)

• the Schiff’s base bond is readily broken and

reformed

• this reversibility is very important in the

biochemical action of this cofactor

N H

CH2OPO3

H3C HO

N H

CH2OPO3

H3C HO

H O

HN

O H

6 Conversion of tryptophan to niacin

7 Conversion of linoleic acid into arachidonic

acid (prostaglandin precursor)

8 Formation of sphingolipids

N H

H3C HO

HN H Lys

N H

H3C HO

HN H

R

O-O R

H3C

HO N

R

H

N H

H3C HO N

R

H H

N H

H3C

HO N

R

H H

H

H+

N H

H3C HO N

Lys

H H

- CO2

Decarboxylation of

amino acids

Important transaminases

• ALT ( alanine aminotransferase)

• formerly known as SGPT (serum glutamate

Important transaminases

• AST (aspartate aminotransferase)

• formerly known as SGOT (serum glutamate

Important decarboxylases

TYROSINE DOPA DOPAMINE EPINEPHRINE

TRYPTOPHAN 5-HT SEROTONIN

HISTIDINE HISTAMINE

GLUTAMIC ACID GAMMA AMINOBUTYRIC ACID (GABA)

CYSTEINE CYSTEINE SULFINIC ACID TAURINE

N C

COO-O

R

alpha-keto acid pyridoxamine phospha

Mechanism for transamination

reaction