Question

Which one of the following enzymes contains $'Mo'$ as a prosthetic group?
A. Phosphatase
B. Dehydrogenase
C. Isomerase
D. Nitrate reductase

Answer 1

A prosthetic group is a cofactor (non peptide) that binds tightly to a protein or enzyme by covalent or non covalent bonds and is not easily removed. They are permanently associated with their proteins or bind with the apoenzyme. Metal ions such as $Co, Mg, Fe, Cu$ and organic molecules such as biotin and $FAD$ are prosthetic groups.

Complete answer:
Option A (Phosphatase): It catalyzes the hydrolysis of its substrate by using water to cleave a phosphoric acid monoester into a phosphate ion and an alcohol and hence is also called a hydrolase. Its cofactors are $M{{g}^{2+}}$ and $Z{{n}^{2+}}$ ions which are metal ions but do not form a prosthetic group.
So option A is not correct.
Option B (Dehydrogenase): It catalyzes oxidation-reduction reactions and uses $NA{{D}^{+}}$,$FAD$ and $NAD{{P}^{+}}$ as cofactors which are coenzymes that bind only during catalysis and are also called redox cofactors. These are not a part of the prosthetic group.
So option B is not correct.
Option C (Isomerase): It catalyzes reactions that transfer functional groups within a molecule to produce isomeric forms and enable structural or geometric changes within a compound. They don’t require any prosthetic group.
So option C is not correct.
Option D (Nitrate reductase): It catalyzes the reduction of nitrates to nitrites by using $Mo$ or molybdenum as its cofactor which falls under the prosthetic group.
So, option D is correct.

Option D is correct.

Note:
Nitrate reductase is a key enzyme for plants, algae, yeasts, and fungi to acquire nitrogen. It is an important enzymatic source of nitric oxide. It helps in $NO$ homeostasis and supplies electrons from $NADPH$ to the molybdoenzymes $NO$ forming nitrite reductase that is responsible for $NO$ synthesis from nitrite. In higher plants, nitrate reduction is highly regulated and its activity is controlled by light, temperature, $pH,C{{O}_{2}},{{O}_{2}}$, water potential and nitrogen source. For example, drought may lead to increased nitrate reductase protein turnover and accelerated $mRNA$ turnover.