Select ALL TRUE statements about enzyme cofactors / coenzymes.

1. The coenzyme NAD+ is derived from the vitamin niacin and is essential for cellular respiration.

2. Coenzymes or cofactors are irreversibly changed during catalysis.

3. Fe and Zn are metals that serve as enzymatic cofactors.

4. Cofactors are optimal, but optional. Organisms can function normally without cofactors.

1. True 2. False 3. True 4. False

Explanation:

1. True NAD+ is a co-enzyme synthesized from Niacin or Nicotinamide, a member of vitamin B complex. NAD+ is an important cofactor in cellular respiration. In the enzyme catalyzed reaction of Glyceraldehyde-3-P dehydrogenase and lactate dehydrogenase, NAD+ is an important cofactor.

2. False. Cofactor and co-enzymes are not irreversible changed. They are reversible. For example in the 5th step of glycolysis, two molecules of NAD+ are reduced to NADH. The conversion of pyruvate to lactate reconvert NADH to NAD+

3. True . Fe, Zn and Mg are enzyme cofactors. Zinc is a cofactor of alcohol dehydrogenase, carboxyl peptidase. Iron is a cofactor of cytochrome oxidase. Magnesium is a cofactor of Hexokinase.

4. False. Cofactors are optimal, and are not optional. Organisms can not function normally without cofactors.

For example when NAD+ is reduced to NADH in glycolysis the availability of NAD+ in the cell is limited. Therefore the absence of a cofactor becomes a bottleneck in the whole reaction process.

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