Post-Translational Modifications & Protein Kinases – Comprehensive Study Notes

Post-Translational Modifications (PTMs)

• Covalent chemical changes introduced into polypeptide chains after translation.
• Serve as reversible molecular switches that expand proteomic diversity and regulate activity, localization, stability and intermolecular interactions.
• Can be single-event (binary switch) or combinatorial ("PTM code" reading/writing).

Canonical PTMs Highlighted in the Lecture

1. Phosphorylation
   • Donor: ATP
   • Example substrate: Glycogen phosphorylase (classic metabolic on/off switch).
   • Enzyme pair: Kinases (writers) / Phosphatases (erasers).

2. Acetylation
   • Donor: Acetyl-CoA
   • Example substrate: Histone tails—controls chromatin compaction and gene expression.
   • Enzyme pair: Acetyl-transferases / De-acetylases (e.g., HDACs).

3. Ubiquitinylation
   • Donor: Pre-activated ubiquitin protein
   • Example substrate: Traf6, an E3 ligase in innate-immune signalling.
   • Enzyme pair: E1–E2–E3 ubiquitin ligases / De-ubiquitinases (DUBs).
   • Marks proteins for degradation, signalling scaffolds, or trafficking.

4. Methylation
   • Donor: S-adenosyl-L-methionine (SAM)
   • Example substrate: FOXO1 transcription factor—modulates DNA binding & longevity pathways.
   • Enzyme: Protein methyl-transferases; often antagonised by demethylases.

5. Myristoylation
   • Donor: Myristoyl-CoA
   • Example substrate: c-Abl tyrosine kinase—lipidation targets it to membranes.
   • Enzyme: N-myristoyl-transferase (NMT).

Structural & Functional Consequences of PTMs

• PTMs create new chemical groups → new surfaces → altered conformations.
• These physical changes translate into modified affinity, catalytic rates, or mechanical properties.

Acetylation Weakens Lateral Contacts in Microtubules (Tubulin Example)

• Cryo-EM study (Eshun-Wilson et al., 2019): compared non-acetylated ($\text{Ac}^0$) vs acetylated ($\text{Ac}^{96}$) $\alpha$-tubulin.
• Lys40 moves $8\,\text{Å}$ farther from the neighbouring M-loop when acetylated.
• Acetyl-K40 buries into hydrophobic core → reduced inter-protofilament contacts → increased flexibility, resistance to mechanical stress.
• Illustrates how a single PTM can tune cytoskeletal stability and intracellular transport.

Phosphorylation – Chemistry & Immediate Effects

• Introduces a $\text{PO}_3^{2-}$ group via a stable yet reversible phospho-ester bond.
• Key physicochemical changes:

1. Adds −2 charge → attracts Lys/Arg/His; repels acidic patches.

2. Supplies up to 3 directional H-bond acceptors/donors → locks new secondary contacts.

3. Serves as a recognition epitope for SH2, 14-3-3, FHA, PTB domains.
   • Residue distribution in eukaryotes: $\text{Ser} = 86\%$, $\text{Thr} = 12\%$, $\text{Tyr} = 2\%$.

Catalysis by Protein Kinases

Why ATP is an Ideal Yet Stable Donor

• $\text{P}_\gamma$–O bond is thermodynamically favourable to cleave (ΔG°′ ≈ −30.5 kJ·mol⁻¹) but kinetically trapped without a catalyst.

Seven Minimal Requirements for Phosphoryl Transfer

1. ATP binding in a conserved pocket.

2. Two $\text{Mg}^{2+}$ ions coordinate $\beta$/$\gamma$ phosphates → increase electrophilicity.

3. Shield negative charge, lowering O–P–O repulsion.

4. Substrate docking – align target hydroxyl.

5. Close approach: $d_{P\cdots O} \le 4.5\,\text{Å}$.

6. General base (Asp/Glu) or local environment increases nucleophilicity of Ser/Thr/Tyr-OH.

7. Release of ADP + reset for next cycle.

Protein Kinase A (PKA) – Historical Paradigm

• First solved kinase structure (early 1990s) → template for >500 human kinases.
• Taylor lab timeline (1980-2021) revealed: activation loop phosphorylation, cAMP-dependent regulatory subunits, allosteric dynamics.
• Continues to be a model for drug design & disease mutations.

Conserved Kinase Domain Architecture

Bilobal Fold

• N-lobe (small, β-rich): 5 β-strands + αC helix; grips ATP via Gly-rich loop (GXGGXXG).
• C-lobe (large, α-rich): hosts catalytic HRD motif, activation segment, substrate peptide groove.
• Inter-lobe cleft = active site → binds ATP & substrate.

Key Structural Elements (PKA Numbering)

• Lys72 (β3): bridges $\beta$-phosphate to αC-helix Glu91 (salt bridge) → aligns catalytic groups.
• Activation segment (Ala Thr Ser-rich loop): contains phospho-Thr197; phosphorylation stabilises active conformation by binding to Arg165 (HRD) & Lys189 (β9).
• P+1 loop: positions substrate residue just C-terminal to phosphosite.

Diversity of the Human Kinome

• ≈ 518 genes (~2 % of genome) encode protein kinases.
• Classified into 10 major groups using four criteria:

1. Kinase-domain sequence similarity.

2. Presence of accessory domains.

3. Structural conservation.

4. Biochemical/functional assays.

Representative Groups

• AGC (PKA/PKG/PKC): cytosolic Ser/Thr kinases triggered by second messengers; require C-tail docking + activation-segment phosphorylation.
• CMGC (CDK, MAPK, GSK, CLK): unique requirement for a primed Tyr either in the activation loop or substrate; substrate motif $\text{S/T-P}$.
• TK (Tyrosine Kinase): large family phosphorylating almost exclusively on Tyr; key in growth-factor signalling.
• Atypical: structures deviate from canonical fold (e.g., PI3K-like kinases, Rio kinases).

Evolutionary Perspective

• Dendrograms (Manning et al., 2002) integrate sequence + 3D structure to trace divergence.
• Structural cores mutate slowly → anchor phylogenetic trees when sequences become saturated.
• Specialisation parallels emergence of new cellular pathways (e.g., immune receptors, neuronal plasticity).

Substrate Recognition & Specificity

• Many kinases read linear consensus motifs flanking the phospho-acceptor.
• PKA: $\text{R-R-x-S/T}$ where $x$ = any residue.
• CDK2: $\text{S/T-P}$ with proline + preference for disordered context.
• Intrinsic disorder near phosphosites enhances accessibility and conformational adaptability.

Beyond the Core Motif

1. Distal docking sites on kinase or substrate increase affinity & accuracy.

2. Scaffold/adaptor proteins (e.g., AKAPs for PKA, MAPK scaffolds) localise reactions to membranes, organelles, or signalling hubs.

3. Sub-cellular targeting domains (PH, C2, FYVE) tether kinases to lipid micro-domains.

Docking-Site Allostery Example – p38α MAPK

• Orange docking groove binds D-motif segments of various substrates/regulators.
• Binding can reshape the remote catalytic cleft (pink) → alters activity & specificity.
• Demonstrates allosteric cross-talk ≈ long-range conformational signalling inside proteins.

Regulation of Kinase Activity

• Despite shared catalytic cores, kinases achieve high signalling specificity through modular regulation.

Accessory Domains

• SH2 / PTB: bind $\text{pTyr}$ motifs; control autoinhibition & recruitment.
• SH3: engage proline-rich sequences.
• PH / C2: interact with lipids (e.g., $\text{PI(3,4,5)P}_3$, $\text{PS}$) to localise kinases near membranes.

Phosphorylation Cascades & Feedback

• Kinases frequently phosphorylate other kinases (MAPK, CDK networks).
• Autophosphorylation of activation loop can be cis (intramolecular) or trans (dimer-mediated).

Conformational Switches Illustrated

(a) External SH2-mediated recruitment (Grb2 to phospho-RTK).
(b) Internal SH2 domain folds back → autoinhibition until displaced.
(c) Activation-segment phosphorylation straightens loop (orange) → opens substrate groove.
(d) Phospho-dependent pseudo-substrate binds catalytic cleft, blocking true substrate (common in PKC).

Numerical & Spatial Benchmarks Mentioned

• Acetyl-K40 shift: $8\,\text{Å}$.
• Catalytic distance requirement: $\le 4.5\,\text{Å}$ between nucleophile O and $\gamma$-P.
• ATP contains three phosphates labelled $\alpha \,\beta \, \gamma$.
• Human kinome size: >500 genes ≈ $2\%$ of genome.
• Phospho-residue distribution: Ser $86\%$, Thr $12\%$, Tyr $2\%$.

Practical & Biomedical Implications

• Mis-regulated PTMs underlie cancers, neurodegeneration, metabolic and immune disorders.
• Kinase inhibitors (e.g., imatinib for BCR-Abl) exploit knowledge of activation loops & docking pockets.
• HDAC inhibitors modulate histone acetylation in epigenetic therapies.
• PROTACs harness ubiquitinylation machinery for targeted protein degradation.

Take-Home Points

• PTMs act as versatile, reversible chemical signals dictating protein fate and function.
• Phosphorylation logic combines chemical charge, structural rearrangement, and modular recognition.
• Kinase domain = conserved engine; regulation derives from contextual domains, PTMs, localisation and scaffolds.
• Evolution has diversified a single catalytic fold into a vast signalling network steering virtually every cellular decision.