Nonenzymatic Protein Functions (1A) - Bio & Biochem

1. What is the nonenzymatic function of chaperone proteins such as Hsp70?

   Assist protein folding and prevent aggregation. Chaperones stabilize nascent polypeptides and refold misfolded proteins, ensuring proper conformation without catalyzing reactions.

2. What is the primary nonenzymatic function of integrins?

   Mediate cell–ECM adhesion and bidirectional signaling. Integrins link extracellular matrix to cytoskeleton, enabling adhesion, migration, and signal exchange for tissue organization.

3. What is the primary nonenzymatic function of transcription factors?

   Bind specific DNA sequences to regulate gene expression. Transcription factors' DNA-binding domains interact with promoters or enhancers, modulating RNA polymerase activity for gene control.

4. What is the nonenzymatic function of albumin in plasma?

   Maintain oncotic pressure and carry hydrophobic molecules. Albumin's abundance in blood regulates osmotic pressure and its binding sites transport lipids, hormones, and drugs.

5. What is the major structural protein of the extracellular matrix in connective tissue?

   Collagen. As the most abundant protein in connective tissues, it provides a scaffold for cells and resists tensile forces.

6. What is the primary nonenzymatic function of cadherins?

   Mediate calcium-dependent cell–cell adhesion. Cadherins form adherens junctions via homophilic interactions, stabilized by calcium, to maintain tissue structure and polarity.

7. What is the primary nonenzymatic function of selectins?

   Mediate transient cell–cell adhesion (leukocyte rolling). Selectins' carbohydrate-binding domains enable weak, reversible interactions for leukocyte recruitment during inflammation.

8. What is the nonenzymatic function of tubulin in eukaryotic cells?

   Form microtubules for structure, transport, and chromosome segregation. Tubulin dimers polymerize into hollow tubes essential for cellular architecture, intracellular transport, and mitotic spindle formation.

9. What is the nonenzymatic function of MHC class I molecules?

   Present endogenous peptides to $CD8^+$ T cells. MHC I displays intracellular peptides on cell surfaces, allowing cytotoxic T cells to detect and eliminate infected or abnormal cells.

10. What is the primary nonenzymatic function of myoglobin?

    Store $O_2$ in muscle cells. Myoglobin's single heme group binds oxygen with high affinity, serving as a reservoir for rapid release during muscle activity.

11. What is the main nonenzymatic function of actin and myosin in muscle?

    Force generation and contraction via filament sliding. In muscle contraction, actin thin filaments and myosin thick filaments interact to produce movement through cross-bridge cycling.

12. What is the primary nonenzymatic role of keratin in epithelial cells?

    Mechanical protection and structural integrity (intermediate filaments). Keratin forms resilient filaments that protect epithelial cells from mechanical stress and maintain tissue architecture.

13. What is the key nonenzymatic function of elastin in tissues?

    Provide elasticity and recoil. Elastin's cross-linked structure allows tissues like arteries and lungs to stretch and return to shape without damage.

14. What is the primary nonenzymatic function of hemoglobin?

    Transport $O_2$ in blood (and assist $CO_2/H^+$ handling). Hemoglobin's heme groups reversibly bind oxygen, enabling efficient delivery to tissues while also modulating pH and carbon dioxide.

15. What is the primary nonenzymatic function of ferritin inside cells?

    Store iron in a soluble, nonreactive form. Ferritin's shell sequesters excess iron ions, preventing oxidative damage and providing a releasable store for cellular needs.

16. What is the nonenzymatic function of dynein and kinesin?

    ATP-driven motor transport along microtubules. These motor proteins use ATP hydrolysis to walk along microtubules, facilitating vesicle and organelle transport in cells.

17. What nonenzymatic role do ligand-gated ion channels perform?

    Open/close to allow ion flux in response to ligand binding. Ligand binding induces conformational changes in these channels, controlling membrane potential and cellular signaling.

18. What is the nonenzymatic function of MHC class II molecules?

    Present exogenous peptides to $CD4^+$ T cells. MHC II presents antigens from extracellular pathogens to helper T cells, coordinating immune responses like antibody production.

19. What is the primary nonenzymatic function of cell-surface receptors (e.g., GPCRs, RTKs)?

    Ligand binding that initiates signal transduction pathways. These receptors detect extracellular signals and transmit them intracellularly, regulating processes like metabolism and growth.

20. What is the primary nonenzymatic function of transferrin in blood?

    Transport $Fe^{3+}$ in plasma. Transferrin binds ferric iron tightly to prevent toxicity while delivering it to cells via receptor-mediated endocytosis.

21. What is the nonenzymatic function of the T cell receptor (TCR)?

    Bind peptide–MHC complexes to initiate T cell signaling. TCRs recognize antigenic peptides on MHC, triggering T cell activation and adaptive immune responses.

22. What is the defining feature of a nonenzymatic protein function?

    A protein role not involving catalysis of a chemical reaction. Enzymatic functions specifically catalyze reactions, whereas nonenzymatic roles encompass all other protein activities like binding or support.

23. What is the main nonenzymatic function of antibodies (immunoglobulins)?

    Bind specific antigens for immune recognition and neutralization. Antibodies' variable regions specifically recognize antigens, facilitating immune responses like opsonization and complement activation.

24. What is the primary nonenzymatic function of structural proteins like collagen?

    Provide tensile strength and structural support to tissues. Structural proteins form frameworks that resist mechanical stress, enabling tissues to withstand tension and maintain integrity.

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