Q.57 Given below are two statements :
Statement I: In all naturally occurring 20 amino acids, both carboxyl and amino groups are bonded to the a-
carbon.
Statement II : According to organic chemistry IUPAC nomenclature, a-carbon is also the C-1 carbon of an
amino acid.
In the light of the above statements, choose the correct answer from the options given below :
1. Both Statement I and Statement II are true
2. Both Statement I and Statement II are false
3. Statement I is true but Statement II is false
4. Statement I is false but Statement II is true
Phototrophs and chemotrophs were energy-based; now amino acids focus on structure—key for protein biochemistry exams. The alpha (α)-carbon defines standard amino acids, central to chirality and Ramachandran plots you’ll analyze.
The Two Statements
Statement I: In all naturally occurring 20 amino acids, both carboxyl and amino groups are bonded to the α-carbon.
Statement II: According to organic chemistry IUPAC nomenclature, α-carbon is also the C-1 carbon of an amino acid.
Options:
-
Both Statement I and Statement II are true
-
Both Statement I and Statement II are false
-
Statement I is true but Statement II is false
-
Statement I is false but Statement II is true
Correct Answer: Option 3 (Statement I True, Statement II False)
Statement I holds for the 20 proteinogenic amino acids: the α-carbon bonds to -NH₂ (amino), -COOH (carboxyl), H, and R-group (side chain). Proline’s amino is secondary (ring-bound), but still α-linked. Statement II fails—in IUPAC, the carboxyl carbon is C-1; α-carbon is C-2.
Standard Amino Acid Structure Table
| Component | Bonds to α-Carbon | Notes & Example |
|---|---|---|
| Amino (-NH₂) | Always (directly or via ring in Pro) | Glycine: H₂N-CH₂-COOH |
| Carboxyl (-COOH) | Always | Alanine: H₂N-CH(CH₃)-COOH |
| Hydrogen (H) | Always (except Gly symmetric) | Enables L-chirality |
| R-group | Varies (H in Gly) | Determines properties (hydrophobic, polar) |
This confirms Statement I across all 20.
Detailed Option Analysis
-
Option 1: Both true
Incorrect. I is correct biochemically, but II misstates IUPAC—carboxyl C is C-1 (named as alkanoic acid), α-C is adjacent C-2. -
Option 2: Both false
Incorrect. Statement I matches all textbooks (e.g., Lehninger Principles); only II errs on nomenclature. -
Option 3: Statement I true, II false
Correct. Biochemical α-carbon convention differs from strict IUPAC chain numbering, a common exam trap. -
Option 4: Statement I false, II true
Incorrect. I is universally true; II reverses numbering.
Key Distinction: Biochemical vs IUPAC Numbering
Biochemistry labels the chiral carbon α for functional emphasis. IUPAC for glycine (aminoacetic acid) numbers carboxyl C as 1, methylene (α) as 2. Visualize:
HOOC¹-CH₂²(NH₂)- (C-1 is carbonyl).
Exam tip: Sketch structures—α-C chirality excludes Gly (achiral). For Ramachandran plots, α-C geometry dictates phi/psi angles.
Practice: Is proline’s carboxyl on α-C? Yes—despite imino ring.
