Cellular Functions
High-Yield Summary
- Five nonenzymatic protein categories: structural, motor, binding, cell adhesion (CAMs), and antibodies.
- Structural proteins (collagen, elastin, keratin, actin, tubulin) are mostly fibrous and give cells/tissue shape and support.
- Motor proteins (myosin, kinesin, dynein) are all ATPases — they convert ATP hydrolysis directly into mechanical movement along a cytoskeletal track.
- Binding proteins stabilize or transport a target WITHOUT chemically modifying it — that's what separates them from enzymes.
- CAMs: cadherins (Ca²⁺-dependent, cell-cell), integrins (ECM-to-cytoskeleton, also signal-sensing), selectins (carbohydrate-binding, weak/transient, immune cell rolling).
- Antibodies = 2 heavy chains + 2 light chains; variable region (tips) binds antigen, constant region directs the immune response.
Key Terms
- Extracellular matrix (ECM)
- Network of proteins outside the cell (e.g., collagen) that gives tissue its structure.
- ATPase
- Enzyme/motor that hydrolyzes ATP to drive a conformational change — defines every motor protein.
- Cadherin
- Calcium-dependent cell adhesion molecule binding identical cadherins on neighboring cells.
- Integrin
- Membrane-spanning CAM linking ECM (outside) to cytoskeleton (inside); also senses external signals.
- Selectin
- CAM binding carbohydrates on other cells; mediates weak, transient immune-cell rolling during inflammation.
- Antigen
- Foreign molecule (on viruses, bacteria, toxins) that triggers antibody production.
- Variable region
- Antigen-binding tips of an antibody's Y-shape; differs antibody to antibody.
- Constant region
- Bulk of the antibody structure; similar across antibodies of the same type, directs immune signaling.
Binding Protein vs. Enzyme
| Binding Protein | Enzyme |
|---|---|
| Stabilizes/transports its target | Converts substrate into product |
| Target leaves chemically unchanged | Target is chemically altered |
Cadherins vs. Integrins vs. Selectins
| Feature | Detail |
|---|---|
| Cadherins | Ca²⁺-dependent; strong, stable cell-cell adhesion (e.g., epithelium) |
| Integrins | Span membrane; link ECM to cytoskeleton; also sense/transmit external signals |
| Selectins | Bind carbohydrates; weak/temporary; mediate immune cell rolling along vessel walls |
Common MCAT Trap
- A binding protein that appears to "change" its target (e.g., by releasing it elsewhere) is still not an enzyme — no covalent change to the target itself occurs.
- Don't confuse motor protein tracks: myosin walks on actin (muscle contraction); kinesin and dynein both walk on microtubules but move in opposite directions/purposes (kinesin — cargo/chromosome transport; dynein — cilia/flagella movement).
- Cadherins vs. integrins vs. selectins are easy to mix up — anchor them by binding partner: cadherin-to-cadherin, integrin-to-ECM/cytoskeleton, selectin-to-carbohydrate.
Quick Recall
What single property defines every motor protein, regardless of its track?
What distinguishes a binding protein from an enzyme, mechanistically?
Which CAM requires calcium, and what does that enable?
What are the two chain types in an antibody, and which region binds antigen?