1.3 Non-Enzymatic Protein Function

Structural Proteins: Extracellular Matrix & Cytoskeleton

Non-enzymatic functional proteins serve critical roles in cellular infrastructure, intracellular transport, immune defense, cell adhesion, and systemic transport. Structural proteins generally possess repetitive secondary structural motifs that assemble into fibrous supramolecular complexes.

1. Collagen

• Function: Primary structural protein of extracellular matrix, connective tissues, bone, tendons, and cartilage, providing immense tensile strength.
• Structure: Tropocollagen is a right-handed triple helix composed of three left-handed alpha-chains. It features a repeating Gly-X-Y motif, where X is predominantly proline and Y is hydroxyproline.
• Role of Glycine: Glycine's small size (single H atom) is sterically mandatory at every third position to fit inside the tightly packed inner axis of the triple helix.
• Vitamin C Cofactor Requirement: Post-translational hydroxylation of proline and lysine residues in the ER lumen is catalyzed by prolyl hydroxylase and lysyl hydroxylase, enzymes requiring Vitamin C (ascorbate) and $\text{Fe}^{2+}$. Vitamin C deficiency prevents hydroxyproline formation, impairing triple helix hydrogen-bonding stability and causing scurvy (bleeding gums, petechiae, fragile capillaries).

2. Elastin & Keratin

• Elastin: Primary component of extracellular matrix in tissues requiring elastic recoil (lungs, large arteries, skin). Unfolded hydrophobic domains are extensively cross-linked by desmosine structures.
• Keratins: Intermediate filament proteins found in epithelial cells, hair, nails, and skin. High cysteine content enables extensive inter-chain disulfide cross-linking, determining mechanical rigidity.

3. Actin & Tubulin (Cytoskeletal Polymers)

• Actin: Monomeric globular actin (G-actin) polymerizes in an ATP-dependent manner into double-stranded filamentous F-actin (microfilaments). F-actin exhibits structural polarity with a fast-growing plus (+ / barbed) end and a slower minus (- / pointed) end, undergoing dynamic treadmilling.
• Tubulin: Globular $\alpha$-tubulin and $\beta$-tubulin dimers polymerize into 13 protofilaments forming a hollow cylinder called a microtubule. Microtubules exhibit polarity (+ end at cell periphery, - end anchored at the centrosome/MTOC). Microtubule assembly is GTP-dependent and demonstrates dynamic instability (rapid growth and shrinkage).

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Motor Proteins & Motility Mechanisms

Motor proteins operate as molecular machines, hydrolyzing ATP to generate conformational changes that drive directional mechanical work along cytoskeletal tracks.

                                   KINESIN
                 (Anterograde Transport toward + End / Periphery)
                                      ----->
 (-) Centrosome / MTOC ========================================= (+) Axon Terminal / Periphery
                                      <-----
                                   DYNEIN
                 (Retrograde Transport toward - End / Soma)

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Oxygen Transport & Storage: Hemoglobin vs Myoglobin

Oxygen transport illustrates non-enzymatic ligand-binding dynamics. Myoglobin and hemoglobin utilize an organic heme prosthetic group (protoporphyrin IX ring with a central $\text{Fe}^{2+}$ ion) to reversibly bind oxygen ($O_2$).

  100 % +---------------------------------------------------------+
        |  Myoglobin (Hyperbolic, P50 = 2.8 mmHg)                 |
        |  /~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                      |
   O2   | /                                                       |
  Sat   |/               Hemoglobin (Sigmoidal, P50 = 26 mmHg)    |
   %    |               _.-'~~~~                                  |
        |            _.-'                                         |
        |         _.-'                                            |
      0 +---------------------------------------------------------+
        0             20             40             60      pO2 (mmHg)

Myoglobin

• Structure: Monomeric protein (1 subunit, 1 heme group) localized in muscle tissue.
• Binding Kinetics: Displays a hyperbolic $O_2$ saturation curve with extremely high affinity ($P_{50} \approx 2.8 \text{ mmHg}$).
• Function: Acts as an oxygen storage reservoir; only releases $O_2$ under severe hypoxic muscle exertion.

Hemoglobin

• Structure: Heterotetramer ($\alpha_2\beta_2$, 4 subunits, 4 heme groups) in erythrocytes.
• Cooperative Binding: Displays a sigmoidal $O_2$ saturation curve ($P_{50} \approx 26 \text{ mmHg}$) driven by positive cooperativity (Hill coefficient $n_H \approx 2.8$). Oxygen binding to one subunit converts remaining subunits from the low-affinity Tense (T) state to the high-affinity Relaxed (R) state.
• Allosteric Effectors & Right-Shift (Bohr Effect): Decreased pH ($H^+$), elevated $pCO_2$, increased temperature, and elevated 2,3-bisphosphoglycerate (2,3-BPG) stabilize the T-state, shifting the curve to the right (promoting $O_2$ unloading in metabolically active tissues).

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Antibodies & Cell Adhesion Molecules (CAMs)

Immunoglobulins (IgG Architecture)

Immunoglobulins (antibodies) are Y-shaped proteins produced by plasma B cells that specifically recognize foreign antigens.

• Structure: Composed of two identical heavy chains (~50 kDa) and two identical light chains (~25 kDa) joined by inter-chain disulfide bonds.
• Fab Region (Fragment, antigen-binding): The two upper arms containing variable domains ($V_H, V_L$) and hypervariable loops (CDRs) that bind specific antigen epitopes.
• Fc Region (Fragment, crystallizable): The stem containing constant domains ($C_H$) that bind complement proteins and Fc receptors on macrophages/neutrophils.

Cell Adhesion Molecules (CAMs)

CAMs are transmembrane proteins that anchor cells to adjacent cells or the extracellular matrix (ECM).

1. Cadherins: $\text{Ca}^{2+}$-dependent cell-to-cell adhesion molecules forming homophilic interactions (e.g., E-cadherin links epithelial cells).
2. Integrins: $\alpha/\beta$ heterodimeric transmembrane receptors linking the intracellular actin cytoskeleton to ECM proteins (fibronectin, laminin); mediate cell signaling and cell-ECM adhesion.
3. Selectins: Transmembrane proteins that bind specific carbohydrate moieties on opposing cell surfaces; mediate leukocyte rolling and extravasation during inflammation.