stays the same, increases, or decreases are choices for first part Why is malonate not a substrate for succinate dehydrogenase?Malonate has two carboxylic acid groups.Malonate only has one methylene group.Malonate lacks a thioester bond that has high transfer potential.Malonate is not large enough to bind to the enzyme. Malonate is a competitive inhibitor of succinate dehydrogenase.COOCH₂COOMalonateDetermine how the concentration of each citric acid cycle intermediate will change immediately after the addition of malonate.The concentration of citrate willThe concentration of isocitrate willThe concentration of a-ketoglutarate willThe concentration of succinyl CoA willThe concentration of succinate willThe concentration of fumarate willThe concentration of malate willThe concentration of oxaloacetate will

The reaction catalyzed by succinate dehydrogenase is given below.Succinate + FAD Fumarate + FADH2In the energy yielding direction of citric acid cycle, the reaction proceeds in the forward direction i.e. the conversion of succinate to fumarate.The structures of malonate and succinate are given below.As you can see, malonate is a structural analogue of succinate. The only difference between the two is that succinate has two methylene (-CH2-) groups while malonate only has one. Succinate is converted to fumarate by removing a hydrogen each from both of the methylene groups of succinate and thereby forming a double bond between the two carbons. The structure of fumarate can be represented as -OOC-CH=CH-COO-.Malonate acts as a competitive inhibitor of succinate dehydrogenase as it is a structural analogue of succinate. It can bind to the succinate binding site of succinate dehydrogenase by replacing succinate, as both malonate and succinate have two carboxylic acid groups (COO- ; deprotonated form of carboxylic acid). Malonate is not a substrate for the reaction because it only has one methylene group while succinate has two. 2 methylene groups are required to undergo the reaction of forming a double bond between the two previous methylene carbons.Malonate having 2 carboxylic acid groups is not the reason for it not being a substrate as succinate also has 2 carboxylic acid groups. So, option 1 is incorrect.The original substrate (i.e. succinate) also does not a high energy thioester bond. So, the lack of thioester bond in malonate is not the reason for it not being a substrate. So, option 3 is incorrect.Malonate does bind to the enzyme at the binding site for succinate. That's how it competitively inhibits the reaction. So, option 4 is incorrect. The term 'immediate' is relative. So, we do not know exactly the time period to be considered here, post the addition of malonate, to determine the change in concentration of the citric acid cycle intermediates.But, the most immediate changes will be observed only in the direct substrate and direct product of the reaction that is inhibited by succinate dehydrogenase. At this time period, the concentration of the rest of the intermediates will remain the same. Malonate inhibits the conversion of succinate to fumarate. So, immediately after the addition of malonate, succinate concentration will increase as it is now not able to get converted to fumarate, while fumarate concentration will decrease. The concentration of citrate will stay the same.The concentration of isocitrate will stay the same.The concentration of -ketoglutarate will stay the same.The concentration of succinyl-CoA will stay the same.The concentration of succinate will increase.The concentration of fumarate will decrease.The concentration of malate will stay the same.The concentration of oxaloacetate will stay the same.Malonate is not a substrate for succinate dehydrogenase because malonate only has one methylene group.

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Biochemistry

Why is malonate not a substrate for succinate dehydrogenase? Malonate has two carboxylic acid groups. Malonate only has one methylene group. Malonate lacks a thioester bond that has high transfer potential. Malonate is not large enough to bind to the enzyme.