# DBT JRF 2022 — Question 128

Exam: DBT JRF 2022 · Section: B · Topic: Biochemistry › Protein Structure - Cysteine & Disulfide Bonds · Difficulty: Medium

## Question

Where is the greatest likelihood of the occurrence of a Cysteine residue in a protein structure?

## Options

- **A.** In the active site pocket
- **B.** In the beta sheets
- **C.** In the turns and loops
- **D.** In the alpha helices

## Answer

Option A

## Explanation

Cysteine residue most likely in active site pockets: Cysteine (Cys, C) has a reactive thiol group (−SH, pKa ~8.3) with unique chemical properties: • Nucleophilic: thiolate (−S⁻) at physiological pH can attack electrophilic substrates. • Redox-active: can be oxidised to sulfinates, sulfenates, or form disulfide bonds. • Metal-coordinating: coordinates Zn²⁺, Fe, Cu in metalloenzymes. Where Cys is found: • ACTIVE SITE POCKETS: Cys serves as a catalytic nucleophile in cysteine proteases (papain, caspases), protein tyrosine phosphatases, ubiquitin-conjugating enzymes, and many other enzymes. This is the most functionally important location. • Disulfide bonds: between Cys residues in the extracellular/oxidising environment, stabilising protein structure. • Structural zinc fingers: Cys coordinates zinc for structural or DNA-binding functions. The active site pocket is the location where Cys is most significantly enriched due to its catalytic roles. Answer: Cysteine is most commonly found in active site pockets where its reactive thiol (−SH) participates in catalysis.

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