Biochemistry, 4th Edition P107 potx

The significance of nonreceptor tyrosine kinase activity to cell growth and transformation is only partially understood, but 1% of all cellular proteins many of which are also kinases are

helices upon hormone binding could reorient the two intracellular domains, giving

rise to guanylyl cyclase activity

The first tyrosine kinases to be discovered were associated with viral transforming

proteins These proteins, produced by oncogenic viruses, enable the virus to

trans-form animal cells, that is, to convert them to the cancerous state A prime example

is the tyrosine kinase expressed by the src gene of Rous or avian sarcoma virus The

protein product of this gene is pp60 v-src(the abbreviation refers to

phosphopro-tein, 60 kD, viral origin, sarcoma-causing) The v-src gene was derived from the

avian proto-oncogenic gene c-src during the original formation of the virus The

cellular proto-oncogene homolog of pp60v-srcis referred to as pp60c-src pp60v-srcis a

526-residue peripheral membrane protein It undergoes two post-translational

modifications: First, the amino group of the NH2-terminal glycine is modified by

the covalent attachment of a myristoyl group (this modification is required for

membrane association of the kinase; see Figure 32.20) Then Ser17and Tyr416are

Binding

site

PKLD

GCD

active site

PKLD

apo

GCD

cGMP GTP

PKLD

GCD

Complex

GCD

ATP

FIGURE 32.19 The rotation mechanism proposed by Misono and colleagues for transmembrane signaling by the ANP receptor ANP binding causes a twist of the two extracellular domains, leading to rotation of the two intracellular domains and activation of guanylyl cyclase activity PKLD is a protein-kinase-like domain and GCD is

a guanylylcyclase domain (Adapted from Misono, K., Ogawa, H., Qiu, Y., and Ogata, C., 2005 Structural studies of the natriuretic peptide receptor: A novel hormone-induced rotation mechanism

for transmembrane signal transduction Peptides 26:957–968.)

Ser17

Tyr416

(a)

(b)

Tyr 527

NH

P

P

P

FIGURE 32.20 (a) The soluble tyrosine kinase pp60v-src

is anchored to the plasma membrane via an N-terminal

myristoyl group (b) The structure of protein tyrosine

kinase pp60 c-src , showing AMP–PNP in the active site (blue, green, red), Tyr 416 (orange), and Tyr 527 (yellow) Tyr 527 is phosphorylated (purple).

phosphorylated The phosphorylation at Tyr416, which increases kinase activity twofold to threefold, appears to be an autophosphorylation On the other hand, phosphorylation at Tyr527is inhibitory and is catalyzed by another kinase known as CSK The significance of nonreceptor tyrosine kinase activity to cell growth and transformation is only partially understood, but 1% of all cellular proteins (many

of which are also kinases) are phosphorylated by these kinases

Soluble Guanylyl Cyclases Are Receptors for Nitric Oxide

Nitric oxide, or NO, a reactive free radical, acts as a neurotransmitter and as a

sec-ond messenger, activating soluble guanylyl cyclase more than 400-fold The cGMP thus produced also acts as a second messenger, inducing relaxation of vascular smooth muscle and mediating penile erection As a dissolved gas, NO is capable of rapid diffusion across membranes in the absence of any apparent carrier mecha-nism This property makes NO a particularly attractive second messenger because

NO generated in one cell can exert its effects quickly in many neighboring cells

NO has a very short cellular half-life (1 to 5 seconds) and is rapidly degraded by nonenzymatic pathways

32.4 How Are Receptor Signals Transduced?

Receptor signals are transduced in one of three ways to initiate actions inside

the cell:

1 Exchange of GDP for GTP by GTP-binding proteins (G proteins), which leads to

generation of second messengers, including cAMP, phospholipid breakdown

prod-ucts, and Ca2

2 Receptor-mediated activation of phosphorylation cascades that in turn trigger ac-tivation of various enzymes This is the action of the receptor tyrosine kinases described in Section 32.3 Protein kinases and protein phosphatases acting as ef-fectors will be discussed in Section 32.5

3 Conformation changes that open ion channels or recruit proteins into nuclear tran-scription complexes Ion channels are discussed in Section 32.7, and the formation

of nuclear transcription complexes was described in Chapter 29

GPCR Signals Are Transduced by G Proteins

The signals of G-protein–coupled receptors (GPCRs) are transduced by GTP-binding proteins, known more commonly as G proteins The large G proteins are hetero-trimers consisting of - (45 to 47 kD), - (35 kD), and - (7 to 9 kD) subunits The

-subunit binds GDP or GTP and has an intrinsic, slow GTPase activity The G 

complex (Figure 32.21, and see Figure 15.19) in the unactivated state has GDP at the

A DEEPER LOOK

Nitric Oxide, Nitroglycerin, and Alfred Nobel

NO is the active agent released by nitroglycerin (see

accompany-ing figure), a powerful drug that ameliorates the symptoms of

heart attacks and angina pectoris (chest pain due to coronary

artery disease) by causing the dilation of coronary arteries

Nitro-glycerin is also the active agent in dynamite Ironically, Alfred

Nobel, the inventor of dynamite who also endowed the Nobel

prizes, himself suffered from angina pectoris In a letter to a friend

in 1885, Nobel wrote, “It sounds like the irony of fate that I should

be ordered by my doctor to take nitroglycerin internally.”

CH2 O

NO2

CH O

NO2

CH2 O

NO2

䊱 The structure of nitroglycerin, a potent vasodilator.

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BiochemistryInteractive to learn more about

the heterotrimeric G-protein complex.

nucleotide site Binding of hormone to receptor stimulates a rapid exchange of GTP

for GDP on G The binding of GTP causes Gto dissociate from Gand to associate

with an effector protein such as adenylyl cyclase (Figure 32.22) Binding of G  (GTP)

ac-tivates adenylyl cyclase The adenylyl cyclase actively synthesizes cAMP as long as G (GTP)

remains bound to it However, the intrinsic GTPase activity of Geventually hydrolyzes

GTP to GDP, leading to dissociation of G(GDP) from adenylyl cyclase and

reassocia-tion with the Gdimer, regenerating the inactive heterotrimeric Gcomplex

The hormone-activated GPCR is a guanine–nucleotide exchange factor (GEF)—

promoting the exchange of GDP with GTP on the G protein—in a manner entirely

similar to the interaction of EF-Ts with EF-Tu(GDP) (pages 969–970) By contrast,

the Gcomplex, which normally acts to inhibit the spontaneous release of GDP

from G (in the inactivated state of the GPCR), is termed a guanine–nucleotide

dissociation inhibitor (GDI). Other proteins may also behave as GEFs and GDIs;

their actions are discussed in Section 32.5

Two stages of amplification occur in the G-protein–mediated hormone response

First, a single hormone-receptor complex can activate many G proteins before the

hormone dissociates from the receptor Second, and more obvious, the G-activated

adenylyl cyclase synthesizes many cAMP molecules Thus, the binding of hormone to

a very small number of membrane receptors stimulates a large increase in

concen-tration of cAMP within the cell The hormone receptor, G protein, and cyclase

con-stitute a complete hormone signal transduction unit.

Hormone-receptor–mediated processes regulated by G proteins may be

stimula-tory or inhibistimula-tory Each hormone receptor interacts specifically with either a

stimu-latory G protein, denoted Gs, or an inhibitory G protein, denoted Gi(Figure 32.22)

Cyclic AMP Is a Second Messenger

Cyclic AMP (denoted cAMP ) was identified in 1956 by Earl Sutherland, who

termed cAMP a second messenger, because it is the intracellular response

pro-voked by binding of hormone (the first messenger) to its receptor Since

Suther-land’s discovery of cAMP, many other second messengers have been identified

(Table 32.1) The concentrations of second messengers in cells are carefully

reg-ulated Synthesis or release of a second messenger is followed quickly by

degra-dation or removal from the cytosol Following its synthesis by adenylyl cyclase,

cAMP is broken down to 5-AMP by phosphodiesterase (Figure 32.23)

FIGURE 32.21 A heterotrimeric G protein (—pink,

—yellow, —blue) docked with a 2 -adrenergic re-ceptor (green) (pdb id  2RH1 and 1GOT).

+

cAMP

-Effector

Stimulatory

2 -Effector

G protein

GTP–GDP exchange

GTP–GDP exchange

G



Adenylyl cyclase

Gi

Gi

Gs

GsGDP

ATP

ACTIVE FIGURE 32.22 Adenylyl cyclase activity is modulated by the interplay of stimulatory

(G s ) and inhibitory (G i ) G proteins Binding of hormones to 1 - and 2 -adrenergic receptors activates adenylyl

cyclase via G s , whereas hormone binding to 2 -adrenergic receptors leads to the inhibition of adenylyl cyclase.

Inhibition may occur by direct inhibition of cyclase activity by G ior by binding of G ito G s Test yourself

on the concepts in this figure at www.cengage.com/login.

Adenylyl cyclase (AC) is an integral membrane enzyme Its catalytic domain, on the cytoplasmic face of the plasma membrane, includes two subdomains, denoted

VC1and IIC2 Binding of the -subunit of Gs(denoted Gs) activates the AC catalytic domain

Alfred Gilman, Stephen Sprang, and co-workers have determined the structure of

a complex of Gs(with bound GTP) with the cytoplasmic domains (VC1and IIC2) of adenylyl cyclase (Figure 32.24) The Gscomplex binds to a cleft at one corner of the

C2domain, and the surface of Gs-GTP that contacts adenylyl cyclase is the same sur-face that binds the G dimer The catalytic site, where ATP is converted to cyclic AMP, is far removed from the bound G protein

cAMP Activates Protein Kinase A

All second messengers exert their cellular effects by binding to one or more target molecules cAMP produced by adenylyl cyclase activates a protein kinase, which is thus

known as cAMP-dependent protein kinase Protein kinase A, as this enzyme is also known,

activates many other cellular proteins by phosphorylation The activation of protein ki-nase A by cAMP and regulation of the enzyme by intrasteric control was described in detail in Chapter 15 The structure of protein kinase A has served as a paradigm for un-derstanding many related protein kinases (see Figure 15.9)

Ras and Other Small GTP-Binding Proteins Are Proto-Oncogene Products

GTP-binding proteins are implicated in growth control mechanisms in higher or-ganisms Certain tumor virus genomes contain genes encoding 21-kD proteins that bind GTP and show regions of homology with other G proteins The first of these

cGMP Guanylyl cyclase Activates protein kinases, regulates ion channels, regulates

phosphodiesterases

Ca2 Ion channels in ER and plasma membrane Activates protein kinases, activates Ca2-modulated proteins

Phosphatidic acid Membrane component and product of PLD Activates Ca2channels, inhibits adenylyl cyclase

Ceramide PLC action on sphingomyelin Activates protein kinases

Nitric oxide (NO) NO synthase Activates guanylyl cyclase, relaxes smooth muscle

Cyclic ADP-ribose cADP-ribose synthase Activates Ca2channels

*IP3is inositol-1,4,5-trisphosphate; PI is phosphatidylinositol; DAG is diacylglycerol; PLC is phospholipase C; PLD is phospholipase D (see Figure 32.26).

TABLE 32.1 Intracellular Second Messengers*

Pyrophosphatase

P P

P 2

O

H2O

H2O H +

O

O

P

O–

O

P O–

O P O–

O

ATP

Adenine

O

–O

O P O–

AMP

O H

H H

O

H H

O

CH2 O

P O–

Cyclic AMP

Adenylyl cyclase Phosphodiesterase

FIGURE 32.23 Cyclic AMP is synthesized by

membrane-bound adenylyl cyclase and degraded by soluble

phosphodiesterase.

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BiochemistryInteractive to explore the

structure and function of adenylyl cyclase.

genes to be identified was found in rat sarcoma virus and was dubbed the ras gene.

Genes implicated in tumor formation are known as oncogenes; they are often

mu-tated versions of normal, noncancerous genes involved in growth regulation,

so-called proto-oncogenes The normal, cellular Ras protein is a GTP-binding protein

that functions in a manner similar to that of other G proteins described previously,

activating metabolic processes when GTP is bound and becoming inactive when GTP

is hydrolyzed to GDP The GTPase activity of the normal Ras p21 is very low, as is

ap-propriate for a G protein that regulates long-term effects like growth and

differenti-ation A specific GTPase-activating protein (GAP) increases the GTPase activity of

the Ras protein Mutant (oncogenic) Ras proteins have severely impaired GTPase

ac-tivity, which apparently causes serious alterations of cellular growth and metabolism

in tumor cells The conformations of Ras proteins (Figure 32.25) in complexes with

GDP are different from the corresponding complexes with GTP analogs such as

GMP–PNP (a nonhydrolyzable analog of GTP in which the -P and -P are linked by

N rather than by O) Two regions of the Ras structure change conformation upon

GTP hydrolysis These conformation changes mediate the interactions of Ras with

other proteins, termed effectors.

G Proteins Are Universal Signal Transducers

A given G protein can be activated by several different hormone-receptor complexes

For example, either glucagon or epinephrine, binding to their distinctive receptor

proteins, can activate the same species of G protein in liver cells The effects are

additive, and combined stimulation by glucagon and epinephrine leads to higher

cytoplasmic concentrations of cAMP than activation by either hormone alone

G proteins are a universal means of signal transduction in higher organisms,

ac-tivating many hormone-receptor–initiated cellular processes in addition to adenylyl

cyclase Such processes include, but are not limited to, activation of phospholipases

(a)

3 – 5

3

1 – 2

Ventral surface

SW II

SW II

SW I

N 239

N 279

W 281

F 379

F 991

R 913

E 917

L 272

F 991

I 207

V 904

N 905

R 232

I 235

N 279

H 989

N 992

Q 236

L 914

Sw 1 IIC2

Gs

N

N N

C

C

C

VC1

(b)

3

3

3

2

2

1

3

ACTIVE FIGURE 32.24 (a) Two views of the complex of Gswith the VC 1 –IIC 2 catalytic domain

of adenylyl cyclase and G s (b) Details of the Gscomplex in the same orientation as the structures in (a) SW-I

and SW-II are “switch regions,” whose conformations differ greatly depending on whether GTP or GDP is bound.

(Courtesy of Alfred Gilman, University of Texas Southwestern Medical Center.)Test yourself on the concepts in this

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(a)

(b)

FIGURE 32.25 The structure of Ras complexed with

(a) GDP (pdb id  1LF5) and (b) GMP–PNP (pdb id 

1LF0) The Ras p21–GMP–PNP complex is the active conformation of this protein A Mg 2  ion (red) is shown

in both structures.

C and A2and the opening or closing of transmembrane channels for K, Na, and

Ca2in brain, muscle, heart, and other organs (Table 32.2) G proteins are integral components of sensory pathways such as vision and olfaction More than 100 dif-ferent GPCRs and at least 21 distinct G proteins are known At least a dozen differ-ent G-protein effectors have been iddiffer-entified, including a variety of enzymes and ion channels

Specific Phospholipases Release Second Messengers

A diverse array of second messengers are generated by breakdown of membrane phospholipids.

Binding of certain hormones and growth factors to their respective receptors triggers

a sequence of events that can lead to the activation of specific phospholipases The

action of these phospholipases on membrane lipids produces the second messengers shown in Figure 32.26

Gs

Gs

Gs

Gi

Gi/Go

Gq

Golf

Transducin (Gt)

TABLE 32.2 G Proteins and Their Physiological Effects

Liver Adipose tissue Kidney Heart muscle

Brain neurons

Smooth muscle cells in blood vessels

Neuroepithelial cells in the nose

Retinal rod and cone cells of the eye

Epinephrine, glucagon Epinephrine, glucagon Antidiuretic hormone Acetylcholine

Enkephalins, endorphins, opioids

Angiotensin

Odorant molecules

Light

Adenylyl cyclase Adenylyl cyclase Adenylyl cyclase Potassium channel

Adenylyl cyclase, potassium channels, calcium channels Phospholipase C

Adenylyl cyclase

cGMP phosphodiesterase

Glycogen breakdown Fat breakdown Conservation of water Decreased heart rate and pumping force

Changes in neuron electrical activity

Muscle contraction, blood pressure elevation Odorant detection

Light detection (vision)

C O

–O

O

O

O C

Action of phospholipases

Phospholipid

Unsaturated fatty acid (arachidonate)

Eicosanoids and

?

Phosphatidylinositol

DAG Inositol phosphates

Phosphatidylcholine

DAG

Sphingomyelin

Ceramide

(a)

(b)

PLD PLA 2

PLC

PLC

FIGURE 32.26 (a) The general action of phospholipase

A 2 (PLA 2 ), phospholipase C (PLC), and phospholipase D

(PLD) (b) The synthesis of second messengers from

phospholipids by the action of phospholipases and

sphingomyelinase (SMase).

Inositol Phospholipid Breakdown Yields Inositol-1,4,5-Trisphosphate

and Diacylglycerol

Breakdown of phosphatidylinositol (PI) and its derivatives by phospholipase C

produces a family of second messengers In the best-understood pathway,

successive phosphorylations of PI produce phosphatidylinositol-4-P (PIP) and

phosphatidylinositol-4,5-bisphosphate (PIP 2 ) Four isozymes of phospholipase C

(de-noted, , , and ) hydrolyze PI, PIP, and PIP2 Hydrolysis of PIP2by phospholipase

C yields the second messenger inositol-1,4,5-trisphosphate (IP 3 ),as well as another

second messenger, diacylglycerol (DAG) (Figure 32.27) IP3is water soluble and

dif-fuses to intracellular organelles where release of Ca2is activated DAG, on the other

hand, is lipophilic and remains in the plasma membrane, where it activates a Ca2

-dependent protein kinase known as protein kinase C (see following discussion).

HUMAN BIOCHEMISTRY

Cancer, Oncogenes, and Tumor Suppressor Genes

The disease state known as cancer is the uncontrolled growth and

proliferation of one or more cell types in the body Control of cell

growth and division is an incredibly complex process, involving

the signal-transducing proteins (and small molecules) described

in this chapter and many others like them The genes that give rise

to these growth-controlling proteins are of two distinct types:

1 Oncogenes: These genes code for proteins that are capable

of stimulating cell growth and division In normal tissues and

organisms, such growth-stimulating proteins are regulated so

that growth is appropriately limited However, mutations in

these genes may result in loss of growth regulation, leading to

uncontrolled cell proliferation and tumor development These

mutant genes are known as oncogenes because they induce the

oncogenic state—cancer The normal versions of these genes

are termed proto-oncogenes; proto-oncogenes are essential for

normal cell growth and differentiation Oncogenes are

domi-nant, because mutation of only one of the cell’s two copies of

the gene can lead to tumor formation Table A lists a few of the

known oncogenes (more than 60 are now known)

2 Tumor suppressor genes: These genes code for proteins

whose normal function is to turn off cell growth A mutation

in one of these growth-limiting genes may result in a protein product that has lost its growth-limiting ability Since the nor-mal products suppress tumor growth, the genes are known as

tumor suppressor genes Because both cellular copies of a tumor

suppressor gene must be mutated to foil its growth-limiting

action, these genes are recessive in nature Table B presents

several recognized tumor suppressor genes

Careful molecular analysis of cancerous tissue has shown that tumor development may result from mutations in several

proto-oncogenes or tumor suppressor genes The implication is that there

is redundancy in cellular growth regulation Many (if not all) tumors

are either the result of interactions of two or more oncogene prod-ucts or arise from simultaneous mutations in a proto-oncogene and both copies of a tumor suppressor gene Cells have thus evolved with overlapping growth-control mechanisms When one is com-promised by mutation, others take over

Proto-Oncogene Neoplasm(s)

Abl Chronic myelogenous leukemia

Myc Burkitt’s lymphoma; carcinoma of lung, breast,

and cervix

H-Ras Carcinoma of colon, lung, and pancreas;

melanoma

N-Ras Carcinoma of genitourinary tract and thyroid;

melanoma

Src Carcinoma of colon

Jun

Several

Fos

Adapted from Bishop, J M., 1991 Molecular themes in oncogenesis Cell 64:235–248;

Croce, C M., 2008 Oncogenes and cancer New England Journal of Medicine 358:502–511.

TABLE A A Representative List of Proto-Oncogenes Implicated

in Human Tumors

Tumor Suppressor

RB1 Retinoblastoma; osteosarcoma; carcinoma of

breast, bladder, and lung

p53 Astrocytoma; carcinoma of breast, colon, and

lung; osteosarcoma

DCC Carcinoma of colon

NF1 Neurofibromatosis type 1

FAP Carcinoma of colon

adrenal cortex

Adapted from Bishop, J M., 1991 Molecular themes in oncogenesis Cell 64:235–248, and Sherr, C J., 2004 Principles of tumor suppression Cell 116:235–246.

TABLE B Representative Tumor Suppressor Genes Implicated

in Human Tumors

}

Activation of Phospholipase C Is Mediated by G Proteins

or by Tyrosine Kinases

Phospholipase C-, C-, and C- are all Ca2-dependent, but the different phos-pholipase C isozymes are activated by different intracellular events Phosphos-pholipase C- is stimulated by G proteins (Figure 32.28) On the other hand, phospholipase C- is activated by receptor tyrosine kinases (Figure 32.29) The domain

organiza-OH HO

OH OH OH

(IP3)

P

HO

OH OH

OH

P

OH OH

P

P

P

1

4 3 2

1

4 3 2

1

4 3 2

FIGURE 32.27 The family of second messengers

pro-duced by phosphorylation and breakdown of

phos-phatidylinositol PLC action instigates a bifurcating

path-way culminating in two distinct and independent

second messengers: DAG and IP 3

Polypeptide hormone

Phospholipase C-

PIP2 IP3+DAG

Ca2+ Receptor

Gq



FIGURE 32.28 Phospholipase C- is activated specifically

by G q , a GTP-binding protein, and also by Ca2.

Polypeptide hormone

Phospholipase C-

Ca 2+ Tyrosine

kinase

Receptor tyrosine kinase(RTK)

P FIGURE 32.29 Phospholipase C- is activated upon

phos-phorylation by receptor tyrosine kinases and by Ca2.

tion of phospholipase C- and C- is shown in Figure 32.30 The X and Y domains

of phospholipase C- and C- are highly homologous, and both of these domains

are required for phospholipase C activation The other domains of these isozymes

confer specificity for G-protein activation or tyrosine kinase activation

Phosphatidylcholine, Sphingomyelin, and Glycosphingolipids

Also Generate Second Messengers

In addition to PI, other phospholipids serve as sources of second messengers

Breakdown of phosphatidylcholine by phospholipases yields a variety of second

messengers, including DAG, phosphatidic acid, and prostaglandins The action

of sphingomyelinase on sphingomyelin produces ceramide, which stimulates

ceramide-activated protein kinase Similarly, gangliosides (such as ganglioside

GM3; see Chapter 8) and their breakdown products modulate the activity of

pro-tein kinases and GPCRs

Calcium Is a Second Messenger

Calcium ion is an important intracellular signal Binding of certain hormones and

signal molecules to plasma membrane receptors can cause transient increases in

cytoplasmic Ca2 levels, which in turn can activate a wide variety of enzymatic

processes, including smooth muscle contraction, exocytosis, and glycogen

metabo-lism (Most of these activation processes depend on special Ca2-binding proteins

discussed in the following section.) Cytoplasmic [Ca2] can be increased in two ways

(Figure 32.31) As mentioned briefly earlier, cAMP can activate the opening of

plasma membrane Ca2channels, allowing extracellular Ca2to stream in On the

other hand, cells also contain intracellular reservoirs of Ca2, within the

endoplas-mic reticulum and calciosomes, small membrane vesicles that are similar in some

ways to muscle sarcoplasmic reticulum These special intracellular Ca2stores are

not released by cAMP They respond to IP3, a second messenger derived from PI

Intracellular Calcium-Binding Proteins Mediate the Calcium Signal

Given the central importance of Ca2as an intracellular messenger, it should not be

surprising that complex mechanisms exist in cells to manage and control Ca2 When

Ca2signals are generated by cAMP, IP3, and other agents, these signals are translated

into the desired intracellular responses by calcium-binding proteins, which in turn

regulate many cellular processes (Figure 32.32) One of these, protein kinase C, is

de-scribed in Section 32.5 The other important Ca2-binding proteins can, for the most

part, be divided into two groups on the basis of structure and function: (1) the

calcium-modulated proteins, including calmodulin, parvalbumin, troponin C, and

many others, all of which have in common a structural feature called the EF hand

(Figure 32.33), and (2) the annexin proteins, a family of homologous proteins that

interact with membranes and phospholipids in a Ca2-dependent manner

PLC-1

-type

X SH2 SH2 SH3 Y

PLC-1

-type

PLC-1

-type

FIGURE 32.30 The amino acid sequences of phospho-lipase C isozymes , , and  share two homologous

domains, denoted X and Y The sequence of -isozyme

contains src homology domains, denoted SH2 and SH3 SH2 domains (approximately 100 residues in length) in-teract with phosphotyrosine-containing proteins (such

as RTKs), whereas SH3 domains mediate interactions with Pro-rich sequences (Adapted from Dennis, E., Rhee, S., Gillah, M., and Hannun, E., 1991 Role of phospholipases in

gener-ating lipid second messengers in signal transduction The FASEB

Journal 5:2068–2077.)

IP3 cAMP

Endoplasmic reticulum

Calciosome

Ca2+

Ca 2+

Ca 2+

Ca 2+

ANIMATED FIGURE 32.31 Cytosolic [Ca2] increases occur via the opening of Ca2channels

in the membranes of calciosomes, the endoplasmic

reticulum, and the plasma membrane See this figure animated at www.cengage.com/login.

HUMAN BIOCHEMISTRY

An intriguing aspect of the phosphoinositide story is the specific

ac-tion of lithium ion, Li, on several steps of PI metabolism Lithium

salts have been used in the treatment of manic-depressive illnesses for

more than 30 years, but the mechanism of lithium’s therapeutic

ef-fects had been unclear Recently, however, several reactions in the

phosphatidylinositol degradation pathway have been shown to be

sensitive to Liion For example, Liis an uncompetitive inhibitor

of myo -inositol monophosphatase (see Chapter 13) Lilevels simi-lar to those used in treatment of manic illness thus lead to the ac-cumulation of several key intermediates This story is far from com-plete, and many new insights into phosphoinositide metabolism and the effects of Lican be anticipated

+ + +

+

+

+

Phospholipase C-

Inositol-1,4,5-P3 Inositol-1,4-P2

Phosphatidylserine

Outside

Inside

Endoplasmic reticulum

Protein kinase C

Ca 2+ /CaM protein kinase

Cellular responses

Cellular responses

Ca2+

Ca 2+

Ca 2+

Inactive target protein

Active target protein

Inactive target protein

Active target protein

P

P

Polypeptide

hormone

Receptor

G protein



GTP GDP

Inositol trisphosphatase

3 1

4

5

6 2

ACTIVE FIGURE 32.32 IP 3 -mediated signal transduction pathways Increased [Ca2] activates protein kinases, which phosphorylate target proteins Ca2/CaM represents calci-calmodulin (Ca2complexed

with the regulatory protein calmodulin) Test yourself on the concepts in this figure at www.cengage.com/ login.

FIGURE 32.33 (a) Structure of uncomplexed calmodulin (pdb id 1LKJ) Calmodulin, with four Ca 2  -binding domains, forms a dumbbell-shaped structure with two globular domains joined by an extended, central helix Each globular domain juxtaposes two Ca 2  -binding EF-hand domains An intriguing feature of these EF-hand domains is their nearly identical three-dimensional structure despite a relatively low degree of sequence

homol-ogy (only 25% in some cases) (b, c) Complex of calmodulin (red) with a peptide from myosin light chain kinase

(blue); (b) side view; (c) top view (pdb id  1QTX).