1 5 pts Explain how GTPbinding proteins work What would you

1. (5 pts.) Explain how GTP-binding proteins work. What would you predict to happen to GTP-binding proteins if

they were bound to GTPS, a form of GTP that cannot be hydrolyzed?

2. (5 pts. ) Describe three properties that define the fluidity of the lipid bilayer? For each property please be clear on what effect it has on membrane fluidity.

Solution

1.

GTP-binding proteins play an important role in the signal transduction pathway for number of hormones and neurotransmitters. The three protein subunits are alpha, beta and gamma. The alpha and gamma subunits are attached to the membrane through lipid molecules like cholesterol and fatty acids. The receptor proteins possess seven transmembrane alpha-helices.

The hormone or neurotransmitter binding to the receptor allows GTP to replace GDP on the alpha subunit. Alpha subunit dissociates from the other two subunits. Further, the alpha subunit, and the beta and gamma subunits travel all along the inner membrane to the ion channels which also act as membrane enzymes. Potassium channels represent the ion channels and membrane enzyme with this activation type is called as adenylyl cyclase. G-protein acts as stimulatory element rather than inhibitory element. The chain of events stops when GTP is hydrolyzed to GDP by the alpha subunit which leads to the association of all the subunits.

If GTP-S replaces the GDP on the G-protein, this protein might show an inhibitory effect on the sequence of events that leads to the signal transduction pathway. Therefore, GTP-S cannot be hydrolyzed by the alpha subunit.

2.

The three properties defining the fluidity of the lipid bilayer are hydrophobic hydrocarbon tails, shape and amphipathic nature, the composition of the lipid bilayer and its temperature.

Research has proved that polar carotenoids affect the membrane fluidity. Unsaturated alkyl chains and cholesterol form effective hydrophobic barriers for the movement of small polar molecules through the membrane. The interaction of cholesterol with the fluid nature of the lipid bilayer fills the free space between lipid molecules and reduces the flexibility of the other lipid chains present in the surrounding. A synthetic bilayer prepared from a phospholipid type from the liquid state to the rigid crystalline state that is called as phase transition and the temperature used for this to take place is low in case the hydrocarbon chains have double bonds or short.