Cysteine is synthesized from –
**Core Concept**
Cysteine is an essential amino acid that plays a crucial role in various physiological processes, including protein synthesis, antioxidant defense, and the regulation of enzyme activity. Its synthesis is tightly regulated, and it can be obtained through the diet or synthesized de novo from other amino acids.
**Why the Correct Answer is Right**
Cysteine is synthesized from methionine through a series of enzyme-catalyzed reactions. The first step involves the conversion of methionine to homocysteine by the enzyme methionine adenosyltransferase. Homocysteine is then converted to cystathionine by the enzyme cystathionine beta-synthase, which requires vitamin B6 (pyridoxine) as a cofactor. Finally, cystathionine is converted to cysteine by the enzyme cystathionase. This pathway is also known as the transsulfuration pathway.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cysteine is not synthesized from serine, although serine can be converted to cysteine in certain microorganisms.
**Option B:** This option is incorrect because cysteine is not synthesized from tyrosine, which is a precursor to other amino acids such as dopamine and norepinephrine.
**Option C:** This option is incorrect because cysteine is not synthesized from tryptophan, which is a precursor to the neurotransmitter serotonin.
**Clinical Pearl / High-Yield Fact**
The transsulfuration pathway is a critical regulator of homocysteine levels in the body, and defects in this pathway can lead to elevated homocysteine levels, which are associated with increased risk of cardiovascular disease and other disorders.
**Correct Answer: D. Methionine**