Study Notes on Protein Phosphorylation and Kinase Interactions

PROTEIN PHOSPHORYLATION

  • Definition: Phosphorylation is the addition of a phosphoryl group (PO₄) to a protein, inducing conformational changes in proteins, such as glycogen phosphorylase, which is the enzyme responsible for releasing glucose from glycogen.

  • Prevalence: Phosphorylation is very widespread and involves approximately 30% of eukaryotic proteins.

  • Process: A phosphoryl group is transferred from ATP (adenosine triphosphate) to a target protein, wherein phosphate groups are removed by specific enzymes.

KINASE FUNCTIONS AND MECHANISMS

  • Kinases: Enzymes that transfer phosphoryl groups to specific target proteins.

  • Typical Residues for Phosphorylation:

    • Serine (Ser)

    • Threonine (Thr)

    • Tyrosine (Tyr)

  • Structure Implications: The presence of a flattened fluorine group mimics the phosphoryl group in transition state structures, facilitating the capture of intermediates.

    • This process involves covalent linkage to oxygens and rearrangements that promote activation.

SMOOTH MUSCLE CONTRACTION

  • Mechanism: Contraction is triggered through activation of protein kinase by calcium-calmodulin (Ca²⁺-CaM).

  • Reaction Steps:

    • Calcium binding: 4extCa2++extCaM<br>ightarrowextCa4extCaM∗4 ext{Ca}^{2+} + ext{CaM} <br>ightarrow ext{Ca}_4 ext{CaM}^*

    • Interaction with MLCK (myosin light chain kinase):

    • extCa<em>4extCaM∗+extMLCKightarrowextCa</em>4extCaM∗ext−MLCK∗ext{Ca}<em>4 ext{CaM}^* + ext{MLCK} ightarrow ext{Ca}</em>4 ext{CaM}^* ext{-MLCK}^*

    • This phosphorylates myosin in the presence of ATP, resulting in muscle contraction.

PROTEIN KINASE ACTIVATORS

  • Intracellular Activators:

    • Cyclic AMP (PKA activation)

    • Calcium-Calmodulin (CaM dependent PK)

    • Cyclic GMP (PKG)

    • Diacylglycerol (PKC)

  • Specific Recognition: Kinases recognize specific amino acid sequences for activation:

    • For Protein Kinase A (PKA): Sequence includes motifs like Arg-Arg-Xxx-Ser/Thr-Val-Xxx.

    • Proline-Directed Kinases: Sequence like Ser-Pro-Lys-Lys.

    • Casein Kinase 2: Recognizes Ser/Thr-Xxx-Xxx-Glu/Asp.

PROTEIN KINOME CLASSIFICATION

  • Groups: Kinases are classified into various families based on structural and functional characteristics:

    • AGC: Includes PKA, PKG, PKC families.

    • CAMK: Calcium-Calmodulin-dependent protein kinases.

    • CK1: Casein kinase 1.

    • CMGC: Contains cyclin-dependent kinase (CDK), mitogen-activated protein kinase (MAPK), glycogen synthase kinase 3 (GSK3), and MLCK families.

    • STE: Sterile kinases.

    • TK: Tyrosine kinase.

    • TKL: Tyrosine kinase-like.

  • Human Kinases: Approximately 520 human protein kinases identified.

PROTEIN PHOSPHATASES

  • Types:

    1. Metal Ion-Dependent Phosphatases: These utilize metal ions like Fe, Mn, Mg, and Zn (e.g., calcineurin).

    2. Cysteine Residue in Active Site Phosphatases: Characterized by their dependence on redox state; active only in reduced conditions.

  • Functionality: Protein phosphatases remove phosphate groups from Ser, Thr, and Tyr side chains, reversing phosphorylation modifications.

IMPACT OF STRESS ON PHOSPHORYLATION

  • Effects of Stress: Stress responses can be beneficial; however, chronic stress poses detrimental effects on cellular activities.

PHOSPHORYLATION REGULATION

  • Factors Triggering Phosphorylation: Key initiators focus on extracellular stimuli (e.g., CaV1.2 channel) and intracellular signaling (e.g., β-adrenergic receptors) leading to increase in cAMP levels.

  • Positive Feedback Loop: Through PKA and CaMKII activated pathways, significant increases in calcium release from RyR2 (ryanodine receptor 2) occur, enhancing muscular contraction and excitation.

PHOSPHOMIMETIC MUTATIONS

  • Definition: A phosphomimetic mutation substitutes a serine with aspartate (negatively charged) to emulate phosphorylation, significantly modifying protein structure and activity (e.g., altering the RyR2 activity).

RECEPTOR TYROSINE KINASES (RTKs)

  • Functional Summary: RTKs require dimerization for activation, except for Insulin Receptor (InsR). Upon ligand binding, it triggers autophosphorylation and downstream signaling pathways — often targeted by inhibitors to impede angiogenesis.

  • Different Domains: Each RTK has unique extracellular domains responsive to various ligands, aligning activity with cellular responses.

PROTEIN INTERACTION DOMAINS

  • Interactions between proteins can resemble molecular Lego; not all pieces are compatible.

  • Common Protein Domains:

    • SH2: Specifically binds phosphotyrosine; recognizes charge and sequence affinity.

    • SH3: Binds proline-rich sequences (PXXP).

    • PDZ: Binds C-terminal ends of proteins, aiding in organization of protein networks.

    • Ankyrin and SAM domains: Typically involved in protein oligomerization, stabilizing structures.