Question

1. What is a multi-enzyme complex? What are the advantages of a multi-enzyme complex?

2. What is the function of each of the three catalytic enzymes that make up pyruvate dehydrogenase? What is the role of prosthetic groups TPP and lipoic acid and which enzyme is each attached to?

3. What are the substrates and the products of the steps in the Citric Acid Cycle where CO₂, NADH, and FADH₂ are produced?

Answer 1

Ans 1: A multi-enzynme complex constitutes of multiple enzymes which are linked to a particular metabolic pathway together as a complex.

Advantages of multi-enzyme complex-1: multienzyme complex has more than one function.

2:multi-enzyme complex shows higher catalytic rate as the subtrate do not diffuse away from the enzyme complex.Minimizes the substrate's entropy for the whole pathway (the one the multienzyme is capable of) making the reaction (catalysis) go faster than a pathway with different separated enzymes.

3:Can be regulated by multiple substrates &/or allosteric effectors increasing the organisms capability of regulating the enzyme's activity (positive and negative regulation).

Ans2:Function of each of the three catalytic enzymes that make up pyruvate dehydrogenase are as follows:

The first subunit E1 removes the co2.

Sununit E2 i.e. dihydrolipoyl transacetylase transfers the remainder of the substrate to coenzyme A, and

subunit E3 i.e. dihydrolipoyl dehydrogenase transfers the H2 that was retained in the preceding steps to NAD+.

The role of prothestic group TPP which is derived from thiamine (vitamin B1), is associated with the E1 subunit of PDH. In cooperation with an aspartate residue in the active site, TPP forms a carbanion, that is, a negative charge on a carbon atom. The TPP carbanion is resonance-stabilized; an electron can move back and forth between the carbanion and neighboring cationic nitrogen.

The role of lipoic acid -this coenzyme is covalently attached to E2, but due to its long, flexible linker can reach into the active sites of adjacent E1 and E3 subunits as well. The lipoic acid group thus guides the substrate from one active site to the next, preventing it from leaving until it has completed the course.

Ans 3:The substrates and the products of the steps in the Citric Acid Cycle where CO2, NADH, and FADH2 are produced are:

The first turn of the cycle are one GTP (or ATP), three NADH, one QH2 and two CO2.

Because two acetyl-CoA molecules are produced from each glucose molecule, two cycles are required per glucose molecule. Therefore, at the end of two cycles, the products are: two GTP, six NADH, two QH2, and four CO2.