Question

Membrane-bound Krebs cycle enzyme is
(a)Fumarase
(b)Cis-aconitase
(c)Succinic dehydrogenase
(d)Malate dehydrogenase

Answer 1

Hint: The Krebs cycle is also known as the citric acid cycle or tricarboxylic acid (TCA) cycle due to the formation of a tricarboxylic compound. The Krebs cycle consists of many enzymes, but the enzyme which is membrane-bound to the mitochondrial inner membrane is an enzyme that is also utilized in the Electron transport system (ETS).

Complete answer:
The enzyme that is membrane-bound in the Krebs cycle is succinic dehydrogenase. It catalyzes the reaction in which succinate is converted into fumarate in the Krebs cycle with the production of ${ FADH }_{ 2 }$. This enzyme is also found in ETS as part of Complex II which is an enzyme complex bound to the inner mitochondrial membrane. Succinate dehydrogenase is also known as succinate-coenzyme Q reductase.

Additional Information: Let us look at the Krebs cycle in detail.
Krebs cycle:
-The Krebs cycle starts with the condensation of the acetyl group with oxaloacetic acid(OAA) to give citric acid, hence it is also known as the citric acid cycle.
-This is followed by the synthesis of isocitric acid from citric acid.
-Successive steps of decarboxylation take place leading to the formation of $\alpha $-ketoglutaric acid and then succinyl-CoA.
-This succinyl-CoA is converted to succinic acid where a molecule of GTP is synthesized and finally it is converted to OAA allowing the cycle to continue.
-There are three points in this cycle where ${ NAD }^{ + }$ is converted to ${ NAD+H }^{ + }$.
-And there is one point where ${ FAD }^{ + }$ is converted to ${ FAD+H }^{ + }$.
So, the correct option is ‘Succinic dehydrogenase’.

Note: -There is a simultaneous synthesis of ATP from ADP during the conversion of GTP to GDP and this is known as substrate-level phosphorylation.
-Ubiquinone which is an electron carrier in ETS receives reducing equivalents from ${ FADH }_{ 2 }$ and this is the function of complex II
-When ${ NADH }_{ 2 }$ is oxidized there is a production of 3 molecules of ATP, but during oxidation of ${ FADH }_{ 2 }$, there is a production of 2 ATP.