**Question:** Vitamin B12 acts as co-enzyme to which one of the following enzymes?

A. Methionine synthase
B. Deoxyribonucleic acid (DNA) synthesis
C. S-adenosylmethionine synthetase
D. Thiamine pyrophosphatase

**Correct Answer:** C. S-adenosylmethionine synthetase

**Core Concept:** Vitamin B12 is a water-soluble vitamin essential for maintaining proper neurological function, DNA synthesis, and red blood cell production. It plays a vital role in various physiological processes, including acting as a co-enzyme for specific enzymes.

**Why the Correct Answer is Right:** Vitamin B12 acts as a co-enzyme for S-adenosylmethionine synthetase (also known as methionine synthase reductase or MTRR). Methionine synthase is an enzyme that catalyzes the conversion of homocysteine to methionine, utilizing vitamin B12 and folate as co-factors. This process is essential for maintaining methionine levels in the body, which in turn contributes to DNA synthesis and red blood cell production.

**Why Each Wrong Option is Incorrect:**

A. Methionine synthase (MS) is the enzyme that utilizes vitamin B12 and folate to convert homocysteine to methionine. While both MTR and MTRR are involved in the same pathway, they are different proteins. MTR is the enzyme that binds vitamin B12, while MTRR is the enzyme that binds reduced form of MTR (methionine synthase) and donates electrons to maintain its active site.

B. Vitamin B12 is involved in DNA synthesis, but the correct answer is MTR (methionine synthase), not DNA synthesis itself. Vitamin B12 acts as a co-enzyme for MTR, which is crucial for DNA synthesis.

C. S-adenosylmethionine synthetase (SAM synthetase) is an enzyme that synthesizes S-adenosylmethionine (SAM) from ATP and homocysteine. SAM is the universal methyl donor, crucial for various biological processes, including DNA methylation and neurotransmitter synthesis.

D. Vitamin B12 plays a role in red blood cell production, specifically in the synthesis of methionine and DNA methylation. However, the correct answer is MTRR (methionine synthase reductase) and not thiamine pyrophosphatase (TPP), which is involved in thiamine pyrophosphate synthesis, not related to Vitamin B12 or Methionine synthase.

**Clinical Pearls:**

Vitamin B12 deficiency can lead to megaloblastic anemia, megaloblastic neuropathy, and impaired DNA synthesis. Methionine synthase (MS) deficiency can result in homocystinuria, an autosomal recessive disorder characterized by elevated homocysteine levels, leading to neurological symptoms and skeletal abnormalities.

**Clinical Pearls:**