Basic amino-acids are all of the following EXCEPT?

UPUMS Nsg officer-2023

Biochemistry & Nutrition

Easy

Appeared in: UPUMS Nsg officer-2023

Explanation

Proline is the correct answer as it is the exception in the group of basic amino acids.
It is classified as an imino acid, not a basic amino acid, because its side chain forms a rigid pyrrolidine ring by bonding back to the alpha-amino group.
This unique structure gives Proline a nonpolar, aliphatic character, which is fundamentally different from basic amino acids that possess proton-accepting, positively charged side chains.

Option A: Lysine is a classic example of a basic amino acid. Its side chain contains a primary amino group (the ε-amino group) which is positively charged at physiological pH.
Option B: Arginine is a basic amino acid. Its side chain features a guanidinium group, which is one of the strongest organic bases and remains positively charged across a wide pH range.
Option C: Histidine is a basic amino acid. Its side chain contains an imidazole ring, which can act as both a proton donor and acceptor near physiological pH, classifying it as basic.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification of amino acids based on chemical properties of their side chains as background academic context rather than a clinical decision trigger.
The chemical properties of amino acid side chains (basic, acidic, polar, nonpolar) are fundamental to protein structure and function. The charge of basic amino acids is crucial for forming salt bridges that stabilize protein 3D structures and for catalytic activity in enzymes.
Proline's rigid ring structure introduces kinks into polypeptide chains, disrupting typical secondary structures like alpha-helices and beta-sheets. This property is vital for the structure of proteins like collagen.
What if the question asked for an amino acid with a hydroxyl group? Then the answer could be Serine or Threonine, which are important for processes like phosphorylation.

How to Approach the Question

First, analyze the question and identify the keyword "EXCEPT". This indicates that three of the options will share a common characteristic, and you must find the one that does not.
The common characteristic is being a "basic amino acid". Recall the definition: an amino acid with a side chain that can accept a proton and is typically positively charged at physiological pH.
Mentally list or recall the common basic amino acids. The three standard basic amino acids are Lysine, Arginine, and Histidine.
Evaluate each option against this knowledge. You will find that Lysine, Arginine, and Histidine all fit the category.
Proline is the remaining option. Recall its unique structure—an imino acid with a ring formed by its side chain and alpha-amino group. This structure is nonpolar, not basic.
Therefore, Proline is the correct answer as it is the exception.

Concept Tested & Keywords

Concept Tested: Classification of amino acids based on chemical properties of their side chains.
Stem keywords: Basic amino-acids
Lead-in keywords: EXCEPT
Negative lead-in flag: The question asks for the option that is NOT a basic amino acid.

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E6 Biochemistry U Satyanarayana— Part 1 (pp 26-420 of 840) p. 36-38

E6 Physiology Ganong 27e Vol 1 Part 1 p. 26-28