• Classification of Amino Acid Alanine:
• Alanine (2-aminopropanoic acid) contains a non-polar, hydrophobic methyl side chain ($-CH_3$).
• Because its hydrocarbon side chain does not participate in hydrogen bonding or ionic interactions, alanine is categorized as a non-polar, aliphatic amino acid.
• Correct Option: Option 1 (Non polar).

• Chemical Structure of Proline:
• Proline: An exceptional proteinogenic amino acid where the aliphatic side chain folds back and bonds directly to the nitrogen of the amino group, forming a rigid five-membered pyrrolidine ring.
• Because it contains a secondary amine ($-NH-$) rather than a primary amino group ($-NH_2$), proline is structurally classified as an imino acid.
• Correct Option: Option 2 (Imino acid).

• Subcellular Site and Enzymology:
• The branched-chain amino acids (Leucine, Isoleucine, Valine) are essential amino acids whose primary catabolism occurs in extrahepatic tissues and hepatocytes within the mitochondria.
• First Step (Transamination):
• The $\alpha$-amino group is removed and transferred to $\alpha$-ketoglutarate by the enzyme branched-chain amino acid aminotransferase (transaminase), yielding glutamate and the corresponding branched-chain $\alpha$-keto acids.
• Second Step (Oxidative Decarboxylation):
• Catalyzed by the mitochondrial branched-chain $\alpha$-keto acid dehydrogenase (BCKDH) complex.
• nalysis:
• (mitochondria; amino; transaminase): Correct.

Correct option: (3) mitochondria; amino; transaminase

• Chemical Reduction of Disulfide Bonds in Proteins:
• In proteins, covalent disulfide linkages ($-S-S-$) formed between cysteine residues stabilize tertiary and quaternary folding.
• $\beta$-Mercaptoethanol (2-Mercaptoethanol / BME) and Dithiothreitol (DTT) are strong chemical reducing agents containing sulfhydryl ($-SH$) groups.
• They reduce covalent disulfide bridges down to free sulfhydryl (thiol) groups:

$$R-S-S-R' + 2\ HS-\text{CH}_2\text{CH}_2\text{OH} \rightarrow R-\text{SH} + R'-\text{SH} + (\text{S}-\text{CH}_2\text{CH}_2\text{OH})_2$$

• Routinely added to SDS-PAGE sample loading buffers to denature proteins into single polypeptide chains.
• nalysis:
• (Mercaptoethanol): Correct.

Correct option: (1) Mercaptoethanol