Pathways Initiated by Plasma Membrane Receptors

On the basis of the signal transduction pathways they initiate, plasma-membrane receptors can be classified into the types listed in Table 7-4 and illustrated in Figure 7-13.

Three notes on general terminology are essential for this discussion. First, the intercellular chemical messengers (hormones, neurotransmitters, and para-crine/autocrine agents), which reach the cell from the extracellular fluid and bind to their specific receptors, are often referred to as first messengers. Second messengers are nonprotein substances that enter the cytoplasm or are enzymatically generated there as a result of plasma-membrane receptor activation and diffuse throughout the cell to transmit signals. They serve as chemical relays from the plasma membrane to the biochemical machinery inside the cell.

The third essential general term is protein kinase. As described in Chapter 4, protein kinase is the name for any enzyme that phosphorylates other proteins by transferring to them a phosphate group from ATP. Introduction of the phosphate group changes the conformation and/or activity of the recipient protein, often itself an enzyme. There are many distinct protein kinases, each type being able to phosphorylate only certain proteins. The important point is that a variety of protein kinases are involved in signal transduction pathways. These pathways may involve long and complex series of reactions in which a particular inactive protein kinase is activated by phosphorylation and then catalyses the phosphorylation of another inactive protein kinase, and so on. At the ends of these sequences, the ultimate phosphorylation of key proteins (transporters, metabolic enzymes, ion channels, contractile proteins, and so on) underlies the cell's response to the original first messenger.

PART TWO Biological Control Systems

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

II. Biological Control Systems

7. Homeostatic Mechanisms and Cellular Communication

© The McGraw-Hill Companies, 2001

PART TWO Biological Control Systems

Change in: Membrane potential and/or Cytosolic [Ca2+]

First messenger hKM

Receptor

Ion channel

Change in: Membrane potential and/or Cytosolic [Ca2+]

(Multiple steps)

Extracellular fluid

Plasma membrane

First messenger i.

Receptor

Tyrosine kinase

Receptor

Tyrosine kinase

Docking protein

CELL'S RESPONSE

Intracellular fluid

Docking protein

Docking protein

(Multiple steps)

CELL'S RESPONSE

First messenger

Receptor

JAK kinase

First messenger

Protein + ATP

I (Multiple steps) CELL'S RESPONSE

Receptor

G Protein

Effector Activates protein (ion channel or enzyme)

Generates

Change in 2nd membrane potential messengers

(Multiple steps)

CELL'S RESPONSE

Was this article helpful?

0 0
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