Figure 716

Example of amplification in the cAMP system.

each of the two subsequent enzyme-activation steps in our example, another hundredfold amplification occurs. Therefore, the end result is that a single molecule of the first messenger could, in this example, cause the generation of 1 million product molecules. This fact helps to explain how hormones and other messengers can be effective at extremely low extracellular concentrations. To take an actual example, one molecule of the hormone epinephrine can cause the generation and release by the liver of 108 molecules of glucose.

How can activation of a single molecule, cAMP-dependent protein kinase, by cAMP be an event common to the great variety of biochemical sequences and cell responses initiated by cAMP-generating first messengers? The major answer is that cAMP-dependent protein kinase has a large number of distinct substrates—it can phosphorylate a large number of different proteins (Figure 7-17). Thus, activated cAMP-dependent protein kinase can exert multiple actions within a single cell and different actions in different cells. For example, epinephrine acts via the cAMP pathway on fat cells to cause both glycogen breakdown (mediated by one phosphorylated enzyme) and tria-cylglycerol breakdown (mediated by another phos-phorylated enzyme).

It must be emphasized that whereas phosphoryla-tion mediated by cAMP-dependent protein kinase activates certain enzymes, it inhibits others. For example, the enzyme catalyzing the rate-limiting step in glyco-gen synthesis is inhibited by phosphorylation, and this fact explains how epinephrine inhibits glycogen synthesis at the same time that it stimulates glycogen breakdown by activating the enzyme that catalyzes the latter response.

PART TWO Biological Control Systems

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

PART TWO Biological Control Systems

Active Transport

Channels

Channels

Active Transport

cAMP-dependent protein kinase

Oo<x

Enzyme 1 Enzyme 2 Enzyme 3 I

Oo<x

Endoplasmic reticulum Protein Synthesis Ca2+ Transport

RNA Synthesis

Enzyme 1 Enzyme 2 Enzyme 3 I

Nucleus

Lipid Glycogen Glycogen breakdown breakdown synthesis

Plasma membrane

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.

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