Principle of Fish Nutrition

• Sparing Mechanism: The dietary requirement for an essential amino acid can be partially replaced or spared by a metabolically related non-essential or semi-essential amino acid.
• Methionine-Cystine Relationship:
• Methionine is an essential sulfur-containing amino acid that serves as a precursor for cystine/cysteine biosynthesis via the transsulfuration pathway (methionine $\rightarrow$ homocysteine $\rightarrow$ cystathionine $\rightarrow$ cysteine).
• Adequate dietary cystine spares methionine because methionine does not need to be diverted to synthesize cysteine, allowing up to $40-50\%$ of the total sulfur amino acid (TSAA) requirement to be met by cystine.
• nalysis:
• (Cystine): Correct. Cystine spares methionine.
• Options 2 & 3 (Menadione, Phylloquinone): Forms of Vitamin K, unrelated to amino acid sparing.

Correct option: (1) Cystine

• Nitrogenous Excretion in Teleosts:
• Teleost (bony) fishes derive a major fraction of their daily metabolic energy from the deamination of amino acids derived from dietary proteins.
• Transdeamination of amino acids primarily via glutamate dehydrogenase (GDH) in hepatocytes releases free ammonia ($NH_3 / NH_4^+$).
• Over $80-90\%$ of total nitrogenous excretion in teleosts occurs across the branchial epithelium (gills) in the form of un-ionized ammonia ($NH_3$) via passive diffusion and Rhesus (Rh) glycoproteins, with small amounts via active $Na^+/NH_4^+$ exchange.
• nalysis:
• (Ammonia): Correct (Teleosts are ammonotelic).
• (Uric acid): Characteristic of birds and reptiles (uricotelic).
• (Urea): Characteristic of elasmobranchs (sharks, rays) and mammals (ureotelic).

Correct option: (2) Ammonia

• Hierarchical Protein Structure:
• Primary Structure: Refers strictly to the linear sequence of amino acids joined together covalently by peptide bonds (amide linkages formed by condensation of $\alpha$-carboxyl and $\alpha$-amino groups).
• Secondary Structure: Regular local spatial conformations ($lpha$-helices, $eta$-pleated sheets) held together by hydrogen bonds.
• Tertiary & Quaternary Structure: Overall 3D spatial conformation stabilized by ionic bonds, hydrophobic interactions, van der Waals forces, and covalent disulfide bonds.
• nalysis:
• (Peptide bonds): Correct. The primary structure is established entirely by peptide linkages.

Correct option: (3) Peptide bonds