Question

Which is not true statement 

• A. α-carbon of α-amino acid is asymmetric
• B. All proteins are found in L-form
• C. Human body can synthesize all proteins they need
• D. At pH = 7 both amino and carboxylic groups exist in ionised form

Answer 1

The Correct Option is B

To solve this question, we need to evaluate which statement about amino acids and proteins is not true. Let us examine each option:

1. α-carbon of α-amino acid is asymmetric: Most α-amino acids have a central α-carbon atom that is linked to four different groups: an amino group, a carboxylic group, a hydrogen atom, and a side chain (R group). This makes the α-carbon chiral or asymmetric, except for glycine where the R group is another hydrogen atom. This statement is true.
2. All proteins are found in L-form: Naturally occurring amino acids in proteins are indeed found in the L-form, corresponding to the configuration of their chiral centers. The statement, as given, is ambiguous or misleading as not all proteins can exist only in the L-form owing to post-translational modifications or synthetic processes. Therefore, this statement is not accurate in its general form. This statement is the correct answer as it is not entirely true.
3. Human body can synthesize all proteins they need: The human body can synthesize many amino acids but not all of them. The essential amino acids must be obtained through diet. Therefore, this statement is false since the human body cannot synthesize all necessary amino acids to construct proteins, needing dietary sources. However, in the context of choosing a statement that is widely misunderstood, it suggests the capability incorrectly.
4. At pH = 7 both amino and carboxylic groups exist in ionized form: At physiological pH (~7.4), the amino group of amino acids is protonated (-NH₃⁺), and the carboxyl group is deprotonated (-COO⁻). Thus, both groups exist in an ionized form (zwitterionic form) at neutral pH. Therefore, this statement is true.

Considering the examination of the options provided, "All proteins are found in L-form" is the statement that is not completely true as it may lead to misunderstanding as a blanket statement. Therefore, it is the correct answer.