Key Biochemical Reactions in Biomolecules (NEET)

1. Peptide Bond Formation: $\text{H}_2\text{N-CHR}_1\text{-COOH} + \text{H}_2\text{N-CHR}_2\text{-COOH} \xrightarrow{-\text{H}_2\text{O}} \text{H}_2\text{N-CHR}_1\text{-CO-NH-CHR}_2\text{-COOH}$

2. Sucrose Hydrolysis (Invertase/$H^{+}$): $\text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \xrightarrow{\text{invertase}} \text{C}_6\text{H}_{12}\text{O}_6 \text{(glucose)} + \text{C}_6\text{H}_{12}\text{O}_6 \text{(fructose)}$

Optical rotation: sucrose (+66.5°) → mixture (-19.8°) = "invert sugar"

3. Disulfide Bond Formation: $\text{2 Cys-SH} \xrightarrow{[\text{O}]} \text{Cys-S-S-Cys} + 2\text{H}^+ + 2e^-$

Reduction (cleavage) by DTT or beta-mercaptoethanol: $\text{Cys-S-S-Cys} + 2\text{RSH} \xrightarrow{\text{DTT}} 2\text{Cys-SH} + \text{RSSR}$

4. Glucose Ring Closure (Mutarotation equilibrium): $\alpha\text{-D-Glucopyranose} \xrightarrow{+\text{H}_2\text{O}} \text{Open-chain D-Glucose} \xrightarrow{-\text{H}_2\text{O}} \beta\text{-D-Glucopyranose}$ [\alpha]_$D^{20}$: +112.2° \rightarrow \text{equilibrium at } +52.7° \leftarrow +18.7°

5. Collagen Hydroxylation (Requires Vitamin C): $\text{Proline (in procollagen)} + \text{O}_2 + \alpha\text{-KG} \xrightarrow{\text{prolyl hydroxylase (+Fe}^{2+}\text{+Vit C)}} \text{Hydroxyproline} + \text{CO}_2 + \text{Succinate}$

6. Vitamin D Activation Pathway: $\text{7-Dehydrocholesterol} \xrightarrow{\text{UV-B}} \text{Vit D}_3 \xrightarrow{\text{liver, CYP27A1}} \text{25-OH-Vit D}_3 \xrightarrow{\text{kidney, CYP27B1}} \text{Calcitriol (1,25-(OH)}_2\text{-D}_3\text{)}$

7. Cellulose Hydrolysis (by cellulase, not humans): $\text{(Glucose)}_n \xrightarrow{\text{cellulase} + \text{H}_2\text{O}} n \text{ D-Glucose}$

$\text{Cellulose: glucose}–[\beta\text{-1,4}]–\text{glucose}–[\beta\text{-1,4}]–... \xrightarrow{\text{NOT by amylase}}$

8. Fehling's Test (Reducing Sugar): $\text{RCHO} + 2\text{Cu}^{2+} + 5\text{OH}^- \xrightarrow{\Delta} \text{RCOO}^- + \text{Cu}_2\text{O}\downarrow + 3\text{H}_2\text{O}$ Brick-red $Cu_{2}O$ precipitate = positive test