8. Match the components of list I with those in list II
List I                                                                          List II
A. Methylation of Lys                                         I. Collagen structure
B. Hydroxylation of proline                            II. Activates genes by modifying histones in chromatin
C. Phosphorylation of tyr                                III. Targets a protein for degradation
D. Poly- ubiquitination of Lys                       IV. Cell signalling

(a) A – I, B – II, C-III, D-IV,
(b) A – I, B – III, C-IV, D – II,
(c) A – II, B-I, C- III, D-IV,
(d) A – II, B-I, C-IV, D-III,

Introduction

Protein modifications are essential post-translational processes that regulate protein function, stability, and interaction. These modifications allow cells to respond rapidly to changes in the environment and maintain cellular homeostasis. In this article, we’ll explore the specific roles of methylation of lysine, hydroxylation of proline, phosphorylation of tyrosine, and poly-ubiquitination of lysine in biological systems.

Matching Protein Modifications with Their Functions

Here’s a breakdown of how different post-translational modifications relate to their specific biological roles:

✅ Correct Match: (d) A – II, B – I, C – IV, D – III

Detailed Explanation

1. Methylation of Lysine (A → II)
   Lysine methylation, particularly on histone tails, is a key epigenetic mechanism. It modifies chromatin structure and regulates gene expression—either activating or repressing transcription depending on the site of methylation. This process plays a critical role in development, differentiation, and cellular response to environmental signals.

2. Hydroxylation of Proline (B → I)
   This modification is vital in stabilizing the triple-helix structure of collagen, the most abundant protein in the human body. Prolyl hydroxylase enzymes add hydroxyl groups to proline residues, enabling the formation of stable hydrogen bonds within the collagen helix—essential for skin, bone, and connective tissue integrity.

3. Phosphorylation of Tyrosine (C → IV)
   Phosphorylation of tyrosine residues is a key event in signal transduction pathways, especially those involving receptor tyrosine kinases (RTKs). When ligands bind to RTKs, they trigger autophosphorylation of tyrosine residues, initiating cascades that regulate cell growth, differentiation, metabolism, and apoptosis.

4. Poly-ubiquitination of Lysine (D → III)
   The attachment of multiple ubiquitin molecules to a lysine residue marks a protein for degradation by the 26S proteasome. This is a tightly regulated process that ensures the removal of damaged, misfolded, or excess proteins—critical for maintaining protein homeostasis and preventing disease.

Conclusion

Understanding protein modifications and their biological roles is fundamental to molecular biology and biotechnology. Each modification serves a precise function, from gene expression and structural support to cell communication and protein turnover. Grasping these processes helps in fields like drug development, disease research, and therapeutic interventions.