Nonenzymatic Protein Functions (1A) - MCAT Biological and Biochemical Foundations of Living Systems

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.

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

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

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

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.

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

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

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.

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.

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

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.

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

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

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

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

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.

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

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

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

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

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.

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.