Post-Translational Modification

Types of Post-Translational Modifications

Common PTMs

Modification	Addition	Function
Phosphorylation	Phosphate group	Signaling, enzyme activation
Glycosylation	Sugar chains	Stability, recognition
Acetylation	Acetyl group	Gene regulation, protein function
Methylation	Methyl group	Signaling, protein interactions
Ubiquitination	Ubiquitin protein	Degradation, signaling
Lipidation	Lipid/fatty acid	Membrane anchoring
Disulfide bond	Cysteine-cysteine	Protein folding, stability

Modification Statistics

• Over 400 types of PTMs identified
• Most proteins undergo multiple PTMs
• PTMs expand proteome diversity 10-100 fold beyond gene count

How PTMs Work

Phosphorylation (Most Common Signaling PTM)

Inactive Enzyme
        ↓
Kinase adds phosphate (ATP → ADP)
        ↓
Active Enzyme (conformational change)
        ↓
Phosphatase removes phosphate
        ↓
Inactive Enzyme

This cycle enables rapid, reversible signal transduction.

Glycosylation

Type	Location	Function
N-linked	Asparagine residues	Protein folding, secretion
O-linked	Serine/threonine	Signaling, stability
Glycosylphosphatidylinositol	C-terminus	Membrane anchoring

PTMs in Peptide Hormone Signaling

Insulin Receptor Signaling

Insulin Binding
        ↓
Receptor Autophosphorylation
        ↓
IRS Protein Phosphorylation
        ↓
PI3K Activation
        ↓
Akt Phosphorylation
        ↓
Glucose Uptake, Metabolic Effects

Growth Hormone Signaling

• GH binding triggers receptor phosphorylation
• JAK2 kinase phosphorylates STAT5
• Phosphorylated STAT5 enters nucleus
• Gene transcription activated

Therapeutic Peptide Modifications

Engineering Better Peptides

Natural peptides are often modified for therapeutic use:

Peptide	Modification	Purpose
Semaglutide	Fatty acid (C18) attached	Albumin binding, extended half-life
Tirzepatide	Fatty acid (C20) attached	Extended half-life
Insulin glargine	Amino acid changes	Altered isoelectric point, slow release
Detemir	Fatty acid attached	Albumin binding

How Fatty Acid Modification Works

Semaglutide (with C18 fatty acid)
        ↓
Injection
        ↓
Fatty acid binds serum albumin
        ↓
Slow release from albumin
        ↓
Protected from proteolysis
        ↓
Extended circulation time
        ↓
Weekly dosing possible

PTMs and Protein Stability

Stabilizing Modifications

Modification	Stability Effect
Disulfide bonds	Locks protein structure
Glycosylation	Protects from proteases
PEGylation	Shields from degradation
Lipidation	Enables albumin binding

Destabilizing Modifications

Modification	Effect
K48 polyubiquitination	Targets for proteasome
Oxidation	Can cause unfolding
Deamidation	Structural instability

PTMs in Disease

Abnormal PTMs

Disease	PTM Abnormality
Cancer	Aberrant phosphorylation cascades
Alzheimer’s	Tau hyperphosphorylation
Diabetes	Impaired insulin signaling phosphorylation
Heart disease	Abnormal protein glycation

Glycation vs Glycosylation

Process	Mechanism	Outcome
Glycosylation	Enzyme-controlled	Normal protein function
Glycation	Non-enzymatic (glucose attack)	Protein damage (AGEs)

HbA1c measures glycated hemoglobin, reflecting average blood glucose.

Analyzing PTMs

Detection Methods

Method	What It Detects
Mass spectrometry	Identifies and locates PTMs
Western blot with PTM-specific antibodies	Specific modifications
Phospho-flow cytometry	Phosphorylation states
Lectin binding	Glycosylation patterns

PTMs in Drug Development

Design Considerations

Native Peptide Analysis
        ↓
Identify Limitations
(short half-life, instability, poor absorption)
        ↓
Select Appropriate PTM Strategy
        ↓
Synthesize Modified Peptide
        ↓
Test Pharmacokinetics/Pharmacodynamics
        ↓
Optimize Modification
        ↓
Clinical Development

Success Stories

Drug	Original Problem	PTM Solution
Semaglutide	GLP-1 half-life: 2 min	Fatty acid → weekly dosing
Insulin lispro	Slow absorption	Amino acid swap → rapid action
Pegfilgrastim	Frequent dosing needed	PEGylation → once per cycle

Frequently Asked Questions

Why can’t we just give natural peptide hormones as drugs?

Natural peptides often have very short half-lives due to rapid proteolysis and clearance. Post-translational modifications like fatty acid attachment or PEGylation protect the peptide and extend its activity, making practical dosing regimens possible.

Do all protein modifications require enzymes?

No. While most PTMs are enzyme-catalyzed (e.g., phosphorylation by kinases), some occur non-enzymatically. Glycation, for example, happens spontaneously when sugars react with proteins, which is why high blood glucose causes protein damage.

Can PTMs be reversed?

Many PTMs are reversible. Phosphorylation is removed by phosphatases, acetylation by deacetylases, and ubiquitination by deubiquitinating enzymes. This reversibility enables dynamic regulation of protein function.