Vitamin C is the co-enzyme for the following hydroxylases EXCEPT
**Core Concept**
Vitamin C (ascorbic acid) plays a crucial role as a co-factor for enzymes involved in the hydroxylation of collagen and the synthesis of certain neurotransmitters. It acts as a reducing agent, facilitating the conversion of certain proline and lysine residues to their hydroxylated forms.
**Why the Correct Answer is Right**
Vitamin C is a co-factor for prolyl hydroxylase (P4H) and lysyl hydroxylase (LH), which are essential for the stabilization and cross-linking of collagen fibers. Prolyl hydroxylase catalyzes the hydroxylation of proline residues, while lysyl hydroxylase hydroxylates lysine residues. These modifications are critical for the formation of triple helices in collagen, which provides strength and elasticity to connective tissue. Vitamin C deficiency leads to impaired collagen synthesis, resulting in scurvy.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because vitamin C is indeed a co-factor for prolyl hydroxylase and lysyl hydroxylase, making it a correct association.
**Option B:** This option is incorrect because vitamin C is not a co-factor for the enzymes involved in the synthesis of catecholamines (dopamine, norepinephrine, and epinephrine). Tyrosine hydroxylase, the key enzyme in this pathway, requires tetrahydrobiopterin (BH4) as its co-factor, not vitamin C.
**Option C:** This option is incorrect because vitamin C is not a co-factor for the enzymes involved in the synthesis of serotonin. Tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, requires tetrahydrobiopterin (BH4) as its co-factor, not vitamin C.
**Option D:** This option is incorrect because vitamin C is not a co-factor for the enzymes involved in the synthesis of melatonin. Tryptophan hydroxylase and arylalkylamine N-acetyltransferase are the key enzymes in melatonin synthesis, but neither requires vitamin C as a co-factor.
**Clinical Pearl / High-Yield Fact**
Vitamin C deficiency can lead to impaired collagen synthesis, resulting in scurvy, a condition characterized by poor wound healing, joint pain, and bleeding gums.
**Correct Answer:** B.