Based on the context given below, answer the following questions:
Glycine is most often used as a buffering agent in biochemical experiments. This diprotic amino acid has a pKa value of 2.4 owing to carboxyl group and 9.6 owing to amino group. Glycine can exist as a zwitterion which can exist in either protonated form (NH₃⁺) or as a free base (–NH₂) because of the reversible reaction:
NH₃⁺ – CH₂ – COOH ⇌ NH₃⁺ – CH₂ – COO⁻ ⇌ NH₂ – CH₂ – COO⁻
Q72.Glycine is highly conserved amino acid because
-
it allows polypeptide chain to make turns
-
it exists as zwitterion
-
its α-carbon is bonded to four different groups
-
it induces polarity
Glycine is highly conserved in proteins due to its unique structural flexibility from a hydrogen side chain, enabling tight turns in polypeptide chains. The correct answer is the first option: “it allows polypeptide chain to make turns.”
Option Analysis
-
A. Correct. Glycine’s side chain is just a hydrogen atom (H), making its α-carbon the least sterically hindered among amino acids. This high conformational flexibility allows glycine to adopt phi/psi angles in Ramachandran plots that other amino acids cannot, facilitating sharp turns, loops, and breaks in secondary structures like alpha-helices—key in collagen and tight protein folds.
-
B. Incorrect. While glycine exists as a zwitterion (NH₃⁺–CH₂–COO⁻) at physiological pH due to its pKa values (2.4 for carboxyl, 9.6 for amino), this property is common to all amino acids and does not explain its conservation across evolution.
-
C. Incorrect. Glycine’s α-carbon bonds to H, –NH₃⁺, –COO⁻, and –H—two identical hydrogens—so it lacks chirality (no four different groups). All other amino acids (except glycine) have four distinct groups, making them chiral (L-form).
-
D. Incorrect. Glycine is non-polar and achiral, inducing no polarity; polar amino acids like serine or asparagine do that via their side chains.
The glycine highly conserved amino acid property stems from its simplest structure—a single hydrogen side chain enabling unique flexibility for polypeptide chain turns in proteins like collagen. Vital for biochemistry students studying protein folding and amino acid properties, this MCQ tests evolutionary conservation reasons amid glycine’s zwitterion behavior and pKa values.
Glycine Structure Basics
Glycine (Gly, G) is the smallest amino acid (formula: NH₂-CH₂-COOH).
-
Side chain: H (no steric bulk).
-
Zwitterion form: NH₃⁺–CH₂–COO⁻ between pKa 2.4–9.6.
-
Achiral α-carbon: Two H atoms prevent optical activity.
This minimal R-group grants maximal rotational freedom, unlike bulkier residues.
Why Highly Conserved?
Evolution favors glycine in tight structural motifs:
-
Polypeptide turns: Accesses “forbidden” Ramachandran angles for beta-turns and loops.
-
Abundant in collagen (35%), disrupting helices for triple-helix coiling.
Zwitterion existence or lack of four different α-carbon groups doesn’t drive conservation.
MCQ Breakdown
See option analysis above. Choice A is correct; others confuse common traits with unique structural roles.
-
