Download MBBS Hormone 3 Lecture PPT

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PERTUSSIS TOXIN

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Protein Kinase

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? In prokaryotic cells, cAMP binds to a specific protein

called catabolite regulatory protein (CRP) that binds
directly to DNA and influences gene expression.

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? By contrast, in eukaryotic cells, cAMP binds to a protein

kinase called protein kinase A (PKA), a heterotetrameric
molecule consisting of two regulatory subunits (R) that

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inhibit the activity of the two catalytic subunits (C) when
bound as a tetrameric complex.

? cAMP binding to the R2 C2 tetramer results in the

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following reaction:


?

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? The R2 C2 complex has no enzymatic activity, but

the binding of cAMP by R induces dissociation of
the R?C complex, thereby activating the latter.

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? The active C subunit catalyzes the transfer of the

phosphate of ATP to a serine or threonine residue in
a variety of proteins.

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? Protein phosphorylation is now recognized as being

a major and ubiquitous regulatory mechanism.
? The effects of cAMP in

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eukaryotic cells are all

thought to be mediated by

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- carbohydrate and fat

protein phosphorylation-

metabolism,

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dephosphorylation,

- enzyme induction

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principally on serine and

- Gene regulation,

threonine residues.

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- synaptic transmission, and
-

cell

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growth

and

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? The control of any of the

replication,

effects

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of

cAMP,

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could be conferred by a

including such diverse

specific protein kinase,

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processes as

by a specific phosphatase,

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? steroidogenesis,

or by specific substrates

? secretion,

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for phosphorylation

? ion transport
? The array of specific

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substrates define a target tissue,

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and are involved in defining the extent of a particular
response within a given cell.

? For example, the effects of cAMP on gene

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transcription are mediated by CREB, the cyclic AMP
response element binding protein.

? CREB binds to a cAMP responsive DNA enhancer

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element (CRE) in its nonphosphorylated state and is a
weak activator of transcription.

? When phosphorylated by PKA, CREB binds the

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coactivator CREB-binding protein CBP/p300 and as a
result is a much more potent transcription activator.


? CBP and the related p300 contain histone

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acetyltransferase activities, and hence serve as
chromatin-active transcriptional coregulators.

? Interestingly, CBP/p300 can also acetylate certain

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transcription factors thereby stimulating their ability
to bind DNA and modulate transcription.
Phosphodiesterases

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? Actions caused by hormones that increase cAMP

concentration can be terminated in a number of ways,
including the hydrolysis of cAMP to 5-AMP by
phosphodiesterases

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? Phosphodiesterases are subject to regulation by their

substrates, cAMP and cGMP; by hormones; and by
intracellular messengers such as calcium, probably

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acting through calmodulin.

? Inhibitors of phosphodiesterase, most notably

methylated xanthine derivatives such as caffeine,

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increase intracellular cAMP and mimic or prolong the
actions of hormones through this signal
cGMP :an Intracellular Signal

? Cyclic GMP is made from GTP by the enzyme

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guanylyl cyclase, which exists in soluble and
membrane-bound forms.

? Each of these enzyme forms has unique physiologic

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properties

? The atriopeptins, a family of peptides produced in

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cardiac atrial tissues, cause natriuresis, diuresis,
vasodilation, and inhibition of aldosterone secretion.

? These peptides (eg, atrial natriuretic factor) bind to

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and activate the membrane-bound form of guanylyl
cyclase
? This results in an increase of

cGMP by as much as 50-fold in

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some cases, and this is thought to mediate the effects

mentioned above.

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? A series of compounds, including nitroprusside,

nitroglycerin, nitric oxide, sodium nitrite, and sodium azide,

all cause smooth muscle relaxation and are potent

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vasodilators.

? These agents increase cGMP by activating the soluble form

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of guanylyl cyclase, and inhibitors of cGMP

phosphodiesterase (the drug sildenafil [Viagra], for example)

enhance and prolong these responses.

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? The increased cGMP activates cGMP-dependent protein

kinase (PKG), which in turn phosphorylates a number of

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smooth muscle proteins leading to relaxation of smooth

muscle and vasodilation.

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