Figure 714

Cyclic AMP second-messenger system. Not shown in the figure is the existence of another regulatory protein, Gi, with which certain receptors can react to cause inhibition of adenylyl cyclase. [Q]

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Homeostatic Mechanisms and Cellular Communication CHAPTER SEVEN

OO II II

P-O-P-O

il P

OH OH

OH

ATP

H2 Adenine O.

cAMP

Phosphodiesterase

Adenine

FIGURE 7-15

Structure of ATP, cAMP, and AMP, the last resulting from enzymatic alteration of cAMP.

kinase A) (Figure 7-14). As emphasized, protein kinases phosphorylate other proteins—often enzymes— by transferring a phosphate group to them. The change in the activity of those proteins phosphorylated by cAMP-dependent protein kinase brings about the response of the cell (secretion, contraction, and so on). Again we emphasize that each of the various protein kinases that participate in the multiple signal trans-duction pathways described in this chapter has its own specific substrates.

In essence, then, the activation of adenylyl cyclase by a G protein initiates a chain, or "cascade," of events in which proteins are converted in sequence from inactive to active forms. Figure 7-16 illustrates the benefit of such a cascade. While it is active, a single enzyme molecule is capable of transforming into product not one but many substrate molecules, let us say 100. Therefore, one active molecule of adenylyl cyclase may catalyze the generation of 100 cAMP molecules. At

Number of molecules 1

Messenger-receptor

Active adenylyl cyclase cAMP

10,000

1,000,000

Active protein kinase

Phosphorylated enzyme

Products

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment