Question

Using biochemistry concepts and connections by Appling answer the following:

Answer 1 or 2

1. Several of the pathways in this module contain examples of enzymes with opposite function controlled through reciprocal allosteric regulation. Pick an example from one of the pathways studied (pentose phosphate pathway, glycolytic pathway) and describe how a specific allosteric effector affects two different enzymes in an opposing manner. Your answer should also include an explanation of why this reciprocal regulation is important for the control of both pathways.

2. Pick one enzyme that is regulated by both an allosteric effector and covalent modification. The effect of the two regulators can be the same or different (for example, both can activate/inhibit the enzyme or one can activate while the other inhibits). Explain the conditions that lead to each type of regulation, how the effect of each makes metabolic sense in the context of the larger pathway, and why both types of regulation are needed for this particular enzyme.

EDITED

Answer 1

1. Allosteric enzyme is an enzyme that when binds to a particular allosteric modulator, changes it's conformation for a ligand binding site to which the ensyme originlly binds. One example of the allosteric enzyme in the glycolytic pathway is the Phosphofructokinse I. This enzyme catalyses one of the most important conversion in the pathway,i.e., conversion of Fructose-6-phosphate to Fructose-1,6- bisphosphate. This enzyme is allosteric in nature because this enzyme's activity can be used to control the increase or decrease iin the rate of glycolysis according to requirement of cell's energy. PFK-1 converts ATP to ADP during the sugar conversion. The allosteric model of symmetry. PFK-1 has two different binding affinities for the ATP. When the cellular ATP/AMP raio is low, when the cell does not have enough energy, the activity of the enzyme increases. But when the cell has enough ATP, the ATP molecules bind to the allosteric site of the PFK-1 enzyme and inhibits the PFK-1 from functioning and progressinig the glycolytic cycle. PFK is also inhibited by phosphoenolpyruvate and citrate. Production of these enzymes at the very end of the glycolytic pathway. PFK1 is allosterically activated by the high levels of AMP. Hence, ATP is the negative allosteric regulator and AMP is positive regulator of the enzyme Phosphofructokinase I.

2. One enzyme which is allosterically affected and covalently modified is the Glucose-6-phosphate dehydrogenase that is present in the oxidative phase of the pentose phosphate pathway. This enzyme is the rate controlling enzyme of the entire pathway. It can be allosterically activated by the presence of NADP+ and inhibited by the presence of NADPH. Glucose-6-phosphate dehydrogenase converts glucose-6-phosphate to  6-phosphoglucono-δ-lactone. When there is an increase in the utilization of NADPH for fatty acid biosynthesis, there is a dramatic increase in the level of NADP+, thus stimulating G6PD to produce more NADPH