Enzyme activities are changed

Proteins will change their shape, and their functioning, if they are modified either covalently or noncovalently. We have seen examples of both types of modification in signal transduction. Protein kinases add phosphate groups to a target protein, and this covalent change alters the protein's conformation. Cyclic AMP binds to target proteins allosterically, and this noncovalent interaction changes the protein's conformation. In both cases, previously inaccessible active sites are exposed, and the target protein goes on to perform a cellular role.

The G protein-mediated protein kinase cascade stimulated by epinephrine in liver cells results in the phosphorylation of two key enzymes in glycogen metabolism, with opposite effects (Figure 15.15):

► Inhibition. Glycogen synthase, which catalyzes the joining of glucose molecules to synthesize the energy-storing molecule glycogen, is inactivated by phosphoryla-tion. Thus the epinephrine signal prevents glucose from being stored in glycogen.

► Activation. Phosphorylase kinase is activated when a phosphate group is added to it. It goes on to stimulate a protein kinase cascade that ultimately leads to the activation by phosphorylation of phosphorylase, the other key enzyme in glucose metabolism. This enzyme liberates glucose molecules from glycogen.

Thus the same signaling pathway inhibits the storage of glucose as glycogen (by inhibiting glycogen synthase) and promotes the release of glucose through glycogen breakdown (by activating glycogen phosphorylase). As we mentioned earlier, the released glucose fuels the ATP-requiring fight-or-flight response to epinephrine.

Different genes are transcribed

Plasma membrane receptors are involved in activating a broad range of gene expression responses. The Ras signaling pathway, for example, ends in the nucleus (see Figure 15.9). The final protein kinase in the cascade, MAPK, enters the nucleus and phosphorylates a leucine zipper protein called AP-1. This activated protein is a transcription factor, and it stimulates the transcription of a number of genes involved in cell proliferation.

As described earlier in this chapter, lipid-soluble hormones can diffuse through the plasma membrane and meet their receptors in the cytoplasm. In this case, binding of the ligand allows the ligand-receptor complex to enter the nucleus, where it binds to hormone-responsive elements at the

15.14 A Signal Transduction Pathway Leads to the Opening of Ion Channels In the signal transduction pathway for the sense of smell, the final effect is the opening of Na+ channels.The resulting influx of Na+ stimulates the transmission of a scent message to a specific region of the brain.

15.14 A Signal Transduction Pathway Leads to the Opening of Ion Channels In the signal transduction pathway for the sense of smell, the final effect is the opening of Na+ channels.The resulting influx of Na+ stimulates the transmission of a scent message to a specific region of the brain.

Phytochrome Molecule
Odorant molecules

Outside of cell |1| Binding of an odorant to its receptor activates P)Odorant a G protein. molecule promoters of a number of genes. In some cases, transcription is stimulated, and in others it is inhibited.

In plants, light acts as a signal to initiate the formation of chloroplasts. Between this signal and response is a transcription-mediated signal transduction pathway. In bright sunlight, red wavelengths are absorbed by a receptor protein called phytochrome. We will say more about this important receptor later in the book, but for now it is important to note only that it is activated by red light. The activated phy-tochrome binds to cytoplasmic regulatory proteins, which enter the nucleus and bind to promoters of genes involved in the synthesis of important chloroplast proteins. Synthesis of these proteins is the key to plant "greening."

Direct Intercellular Communication

Up to now, we have described how signals from a cell's environment can influence that cell. But the environment of a cell in a multicellular organism is more than the extracellular medium. Most cells are in contact with their neighbors. In Chapter 5, we described how cells adhere to one another by recognition proteins protruding from the cell surface. There are also specialized cell junctions, such as tight junctions and desmosomes, that help "cement" cells together (see Figure 5.6).

However, as we know from our own neighbors (and roommates), just being in proximity does not necessarily mean that there is functional communication. Neither tight junctions nor desmosomes are specialized for intercellular communication. In this section, we look at the specialized junctions between cells that allow them to signal directly one another directly. In animals, these structures are gap junctions; in plants, they are plasmodesmata.

2| The G protein activates the synthesis of cAMP by adenylyl cyclase.

cAMP activates the opening of ion channels.

Outside of cell |1| Binding of an odorant to its receptor activates P)Odorant a G protein. molecule

2| The G protein activates the synthesis of cAMP by adenylyl cyclase.

cAMP activates the opening of ion channels.

Effector cAMP-gated protein channel

^ Changes in ion concentrations inside the cell send a signal to a specific area of the brain, which perceives the signal as a scent.

Signal to brain

Effector cAMP-gated protein channel

Inside of cell

^ Changes in ion concentrations inside the cell send a signal to a specific area of the brain, which perceives the signal as a scent.

Signal to brain

Outside of cell

O Epinephrine

\ Epinephrine 1 ^receptor

Plasma membranes

Activated G protein subunit

O Epinephrine

\ Epinephrine 1 ^receptor

Activated G protein subunit

|ll Phosphorylation, induced by epinephrine binding, inactivates glycogen synthase, preventing glucose from being stored as glycogen.

Inactive protein kinase A

|ll Phosphorylation, induced by epinephrine binding, inactivates glycogen synthase, preventing glucose from being stored as glycogen.

Activated adenylyl cyclase

Active glycogen synthase

Inactive protein kinase A

Inactive glycogen synthase

Activated adenylyl cyclase

Active glycogen synthase

Inactive glycogen synthase

Active protein kinase A

Inactive phosphorylase kinase Active phosphorylase kinase

Inactive glycogen phosphorylase

2t Phosphorylation , activates glycogen phosphorylase, releasing stored glucose molecules from glycogen.

Active glycogen phosphorylase

Glycogen

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Essentials of Human Physiology

Essentials of Human Physiology

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