**Question:** The formation of glycine takes place by transamination of

A. alanine
B. aspartate
C. serine
D. cysteine

**Core Concept:** Transamination is a key pathway in amino acid biosynthesis and catabolism. In this process, an amino acid donates an α-keto acid group to an amino group of an α-ketoglutarate or pyruvate, forming a new amino acid and α-ketoglutarate or pyruvate.

**Why the Correct Answer is Right:** Glycine is an essential amino acid, found in proteins and plays a crucial role in various physiological processes. In the transamination of glycine, alanine acts as the amino donor, forming α-ketoglutarate and glycine. The correct answer is D, cysteine, because cysteine is an essential amino acid and its biosynthesis involves transamination with α-ketoglutarate.

**Why Each Wrong Option is Incorrect:**

A. Alanine (option A) is incorrect because it is the amino acid donating the α-ketoglutarate in the transamination process, not glycine itself.
B. Aspartate (option B) is incorrect because it is another amino acid involved in transamination, but it does not participate in glycine biosynthesis.
C. Serine (option C) is incorrect as it does not participate in glycine biosynthesis via transamination.

**Clinical Pearl:** Transamination reactions are essential for maintaining amino acid homeostasis in the body. They help regulate amino acid levels and facilitate the interconversion between different amino acids. Understanding transamination pathways is crucial for studying the chemistry and physiology of amino acids in the context of protein synthesis, degradation, and transport.