AP Bio: Chp.11 Cell Communication

G-protein-linked receptors: vary in their binding sites and for recognizing different G-proteins. Most have seven alpha helices penetrating the plasma membrane. When a G-protein-linked receptor binds with a signal or ligand, it becomes activated and interacts with a G-protein. Examples: Yeast mating factors, epinephrine, neurotransmitters and other hormones.

G-protein-linked receptors activate G proteins. G proteins are found in the plasma membrane. They are activated when the G-protein-linked receptors causes GTP to replace GDP on the G-protein.

The activated G-protein moves laterally to interact with a given enzyme also located in the plasma membrane This causes a certain biochemical pathway to be activated. Once this occurs, the G-protein cleaves a phosphate group from GTP and makes it GDP and thus the G-protein becomes inactivated. These biochemical pathways include developmental pathways, sensory perception in vision and smell.

Note-Bacteria that often cause disease by secreting toxins. These toxins may interfere with G-protein functions. 60% of all medicines exert their effect by influencing G-protein pathways.

Which molecule would be considered a ligand if it were small? A) A B) B C) C D) D E) E

A The molecule that attaches to the G-linked receptor protein is the ligand. A ligand is a small molecule that attaches to the large molecule. In this case A is considered to be the ligand if it were small.

b. Tyrosine-Kinase receptors- when activated can activate more than one signal- transduction pathway at one time. Example: cell reproduction requires a number of biochemical pathways to be activated at once. The tyrosine-kinase receptor in the inactive form are two separate polypeptide chains forming alpha helices that span the membrane. Inside the cell, the two polypeptides have tyrosines on tails that extend into the cytoplasm. Both have a receptor site for a signal or ligand. Once both receptors are activated, the polypeptides move toward one another and bond with one another to form a dimer.

The dimer is phosphorylated at every tyrosine by ATP. The receptor is activated & can trigger a number of transduction-signal pathways. This may include the transfer of a phosphate to another enzyme. Triggering a number of these pathway is the main difference between this receptor and the G-protein-linked receptor.

The activation of receptor tyrosine kinases is always characterized by A) dimerization and phosphorylation. B) IP 3 binding. C) a phosphorylation cascade. D) GTP hydrolysis. E) channel protein conformational change.

A The molecule has two separated halves that joint together once the ligands attach to the receptor site. Once joined, this molecule with two identical halves is called a dimer. On the molecule there are places where a phosphate group will attach. This place of attachment has tyrosine located there.

c. Ligand-gated ion channels. are protein pores on plasma membrane that open or close in response to a chemical signal allows or prevents the flow of ions into or out of the cell. Examples include Na + or Ca ++. Once the signal attaches to the gated channel, the protein changes shape & a pore opens; allowing for an immediate change in the concentration of ions.

d. Intracellular receptors- NOT found in plasma membrane, they are dissolved in cytoplasm. Signal must be lipid soluble to make into the cytosol. Signals are usually steroid hormones, thyroxin, & nitric oxide. Example: Testosterone enters the cell & binds with a receptor protein found in the cytoplasm. This complex moves to the nucleus where is acts a transcription factor & helps transcribe certain genes. Only targeted cells have intracellular receptors.

Binding of a signal molecule to which type of receptor leads directly to a change in the distribution of anions and/or cations on opposite sides of the membrane? A) receptor tyrosine kinase B) G-protein-linked receptor C) phosphorylated receptor tyrosine kinase dimer D) ligand-gated ion channel E) intracellular receptor

D The ligand-gated channel as illustrated above leads directly to a change in the distribution of anions and cations.

Lipid-soluble signal molecules, such as testosterone, cross the membranes of all cells but affect only target cells because A) only target cells retain the appropriate DNA segments. B) intracellular receptors are present only in target cells. C) most cells lack the Y chromosome required. D) only target cells possess the cytosolic enzymes that transduce the testosterone. E) only in target cells is testosterone able to initiate the phosphorylation cascade leading to activated transcription factor.

B Lipid-soluble signal molecules, such as testosterone, cross the membranes of all cells but affect only target cells because intracellular receptors are present only in target cells.

Transduction: After reception, the cell needs to process the signal, this is a multi-step process. Usually the molecules that process the signal are proteins that are activated by phosphorylating them. Protein kinases are proteins that transfer phosphate groups from ATP to another protein. Serine and threonine are two amino acids that receive the phosphate group on the receiving protein kinase. This cascading affect increases the number of proteins activated, amplifying the effect very quickly.

A biochemical pathway usually has a cascading effect where one protein kinase will activate another protein kinase by transferring a phosphate group on to it. These protein kinases are extremely important. Approximately 1% of all our genes code for various protein kinases. Protein phosphatase is a protein that removes a phosphate group from proteins and can deactivate a protein kinase.

Phosphorylation cascades involving a series of protein kinases are useful for cellular signal transduction because A) they are species specific. B) they always lead to the same cellular response. C) they amplify the original signal manifold. D) they counter the harmful effects of phosphatases. E) the number of molecules used is small and fixed.

C Phosphorylation cascades involving a series of protein kinases are useful for cellular signal transduction because they amplify the original signal manifold.

Between the receptor molecule that is activated and signal- transduction pathway that is to be activated, there may be a second messenger. Note-The hormone or signal that attaches the to the receptor site is called the first messenger. The most common secondary messengers include: a. cyclic AMP b. Calcium ions and inositol triphosphate. Stop day 2