Nonenzymatic Protein Function and Protein Analysis (3)

MCAT Biochemistry Review 2025-2026

Structural Proteins

compose the cytoskeleton, anchoring proteins, and much of the extracellular matrix. They are generally fibrous in nature

Collagen, Elastin, Keratin, Actin, and Tubulin

most common structural proteins

Motor Proteins

have one or more heads capable of force generation through a conformational change. They have catalytic activity, acting as ATPases to power movement. Muscle contraction, vesicle movement within cells, and cell motility are the most common applications of these

Myosin, Kinesin, and Dynein

common examples of motor proteins

Binding Proteins

bind a specific substrate, either to sequester it in the body or hold its concentration at steady state

Cell Adhesion Molecule (CAM)

allow cells to bind to other cells or surfaces

Cadherins

are calcium-dependent glycoproteins that hold similar cells together

Integrins

have two membrane-spanning chains and permit cells to adhere to proteins in the extracellular matrix. Some also have signaling capabilities

Selectins

allow cells to adhere to carbohydrates on the surfaces of other cells and are most commonly used in the immune system

Antibodies (Immunoglobulins, Ig)

are used by the immune system to target a specific antigen

Antigen

may be a protein on the surface of a pathogen (invading organism) or a toxin. Contain a constant region and a variable region

Variable

this region in an immunoglobulin is responsible for antigen binding

Antibody

two identical heavy chains and two identical light chains form a single _________; they are held together by disulfide linkages and noncovalent interactions

Ion Channels

can be used for regulating in flow into or out of a cell

Ungated Channels

are always open

Voltage-gated Channels

are open within a range of membrane potentials

Ligand-gated Channels

open in the presence of a specific binding substance, usually a hormone or neurotransmitter

Enzyme-linked Receptors

participate in cell signaling through extracellular ligand binding and initiation of second messenger cascades

G Protein-couple Receptors

have a membrane-bound protein associated with a trimeric G protein. They also initiate second messenger systems

Ligand

__________ binding engages the G protein

GDP; GTP

_____ is replaced with _____

Alpha; Beta and Gamma

the ______ subunit dissociates from the _______ subunits

Adenylate Cyclase or Phospholipase C

the activated alpha subunit alters the activity of _________

GTP; GDP

______ dephosphorylated to _____; the alpha subunit rebinds to the beta and gamma subunits

Electrophoresis

uses a gel matrix to observe the migration of proteins in response to an electric field

Native PAGE

maintains the protein’s shape, but results are difficult to compare because the mass-to-charge ratio differs for each protein

SDS-PAGE

denatures the proteins and masks the native charge sot hat comparison of size is more accurate, but the functional protein cannot be recaptured from the gel

Isoelectric focusing

separates proteins by their isoelectric point (pI); the protein migrates toward an electrode until it reaches a region of the gel where pH = pI of the protein

Chromatography

separates protein mixtures on the basis of their affinity for a stationary phase or a mobile phase

Column Chromatography

uses beads of a polar compound, like silica or alumina (stationary phase), with a nonpolar solvent (mobile phase)

Ion-exchange Chromatography

uses a charged column and a variably saline eluent

Size-exclusion Chromatography

relies on porous beads. Larger molecules elute first because they are not trapped in the smaller pores

Affinity Chromatography

uses a bound receptor or ligand and an eluent with free ligand or a receptor for the protein of interest

X-ray Crystallography

protein structure is primarily determined through _________ after the protein is isolated, although NMR can also be used

Edman Degradation

amin acid composition can be determined by simple hydrolysis, but amino acid sequencing requires sequential degradation, such as this

BCA Assay, Lowry Reagent Assay, and Bradford Protein Assay

each test for protein and have different advantages and disadvantages

Bradford Protein Assay

is the most common and uses a color change from brown-green to blue