1,800+ MCAT terms, defined.

2,3-BPG (2,3-bisphosphoglycerate) is made in red blood cells and binds deoxyhemoglobin, stabilizing the T state and lowering O₂ affinity. More BPG shifts the oxygen-dissociation curve right (higher P50), so hemoglobin unloads more O₂ in...

The Hill coefficient (n) measures cooperativity of ligand binding to a protein with several binding sites. n = 1 means noncooperative binding (hyperbolic curve, as in myoglobin), n > 1 means positive cooperativity (sigmoidal curve, as in...

An isomerase is an enzyme that converts a molecule into one of its isomers by rearranging atoms within that same molecule, so substrate and product share a molecular formula. Glycolysis is the usual setting: phosphoglucose isomerase and...

Simple diffusion is passive movement of a solute down its concentration gradient directly through the lipid bilayer, with no transport protein or ATP. Small nonpolar molecules like O₂ and CO₂ cross this way, and the rate rises in proport...

A micelle is a spherical cluster of amphipathic molecules, such as soaps, detergents or bile salts, in water, with polar heads facing the water and nonpolar tails packed inside. It forms by the hydrophobic effect, which raises the entrop...

An oxidoreductase is an enzyme that catalyzes an oxidation-reduction reaction: one substrate loses electrons while another gains them, often with a cofactor such as NAD⁺ or FAD. It is EC class 1, and the testable skill is spotting one fr...

An α-helix is a secondary structure in which the polypeptide backbone coils into a right-handed spiral held by hydrogen bonds between backbone C=O and N–H groups four residues apart. Testable point: the R groups point outward, and prolin...

Nucleotide excision repair removes bulky, helix-distorting DNA damage, such as UV-induced thymine dimers, by cutting out a short stretch of the damaged strand. DNA polymerase refills the gap using the intact strand as template, and ligas...

Substrate concentration, [S], is the molar amount of substrate available to an enzyme. For a typical Michaelis-Menten enzyme, rate rises with [S] in a hyperbola and levels off at Vmax once the active sites are saturated. Km is the [S] at...

Splicing is the removal of introns from eukaryotic pre-mRNA and the joining of the remaining exons into mature mRNA, done in the nucleus by the spliceosome. Most testable: alternative splicing lets one gene make several different proteins.

Cooperativity is when ligand binding at one site on a multisubunit protein changes the affinity of its other sites for that ligand. In positive cooperativity, as with hemoglobin and O₂, this gives a sigmoidal binding curve instead of the...

DNA sequencing determines the order of nucleotides in a DNA molecule. The classic MCAT method is Sanger (chain-termination) sequencing, where dideoxynucleotides lacking a 3′-OH stop synthesis, so the sizes of the resulting fragments reve...

An acyl-CoA is a fatty acid joined to coenzyme A by a high-energy thioester bond, the activated form that cells oxidize or build into lipids. Most testable: activating a fatty acid costs 2 ATP equivalents (ATP → AMP + PPi), and long-chai...

Cadherins are Ca²⁺-dependent transmembrane proteins that bind one cell to another, usually by binding the same cadherin type on the neighboring cell. They anchor adherens junctions (via actin) and desmosomes (via intermediate filaments)....

The Michaelis-Menten model describes how the initial rate of a simple, non-allosteric enzyme rises with substrate concentration, giving a hyperbola that levels off at Vmax. Km is the [S] at which v = ½Vmax, and a lower Km often means tig...

Isoelectric focusing is a gel electrophoresis method that separates proteins by their isoelectric point (pI) along a pH gradient. Each protein migrates until the surrounding pH equals its pI, where its net charge is zero and it stops. Se...

Debranching enzyme removes the α-1,6 branches of glycogen during glycogenolysis. It transfers a few glucose residues from a branch to another chain, then hydrolyzes the last α-1,6-linked glucose, releasing free glucose rather than glucos...

Phosphorylation is the covalent addition of a phosphate group, usually from ATP, to a molecule. In proteins, a kinase typically adds it to the OH of serine, threonine, or tyrosine, and a phosphatase removes it. It can activate or inhibit...

A pKa is the negative log of an acid's Ka, and it equals the pH at which a group is half protonated and half deprotonated. Below the pKa the protonated form predominates, and above it the deprotonated form does. A lower pKa means a stron...

tRNA charging is the ATP-driven attachment of a specific amino acid to its matching tRNA by an aminoacyl-tRNA synthetase, forming aminoacyl-tRNA. Most testable: the synthetase, not the ribosome, sets which amino acid is used, so a mischa...

The 3' poly-A tail is a string of roughly 100–250 adenine nucleotides added to the 3' end of eukaryotic pre-mRNA after transcription. It protects the mRNA from degradation and aids export and translation. It is added by poly-A polymerase...

RNA primers are short RNA strands made by primase that pair with the DNA template and give DNA polymerase a free 3′-OH to extend, since it cannot start a strand on its own. The lagging strand needs a new primer for each Okazaki fragment.

A glycolipid is a membrane lipid with a carbohydrate attached by a glycosidic bond, most often on a ceramide (sphingolipid) backbone. In animal cells it sits in the outer leaflet of the plasma membrane with its sugar facing the extracell...

Native PAGE is polyacrylamide gel electrophoresis run without SDS or reducing agents, so proteins stay folded and keep their subunit associations. They separate by net charge, size, and shape together, unlike SDS-PAGE, which separates ma...

Inducible describes a gene, enzyme, or operon that is expressed at a low level until a specific molecule, the inducer, turns it on. The classic MCAT example is the lac operon, where allolactose, a lactose derivative, inactivates the repr...

1/Km is the reciprocal of the Michaelis constant, the substrate concentration giving half of Vmax. A larger 1/Km usually signals higher apparent substrate affinity, though this is approximate. On a Lineweaver-Burk plot, the x-intercept i...

1/Vmax is the reciprocal of an enzyme's maximum velocity. It is the y-intercept of a Lineweaver-Burk plot (1/v₀ versus 1/[S]). A higher intercept means a lower Vmax, so competitive inhibition leaves the intercept unchanged and other inhi...

An acetyl group is the two-carbon acyl group CH₃–C(=O)–, carried in cells on coenzyme A as acetyl-CoA, a high-energy thioester. Acetyl-CoA from pyruvate, fatty acids and ketogenic amino acids enters the citric acid cycle by combining wit...

A phosphatase is a hydrolase that removes a phosphate group from a substrate by adding water. It is not the reverse of a kinase reaction, because it does not make ATP. Dephosphorylation can activate or inhibit a protein, depending on whi...

An activator is a molecule that increases the activity of an enzyme (an allosteric activator binds away from the active site) or the transcription of a gene (a DNA-binding protein that helps recruit RNA polymerase). Most testable: an all...

Michaelis-Menten kinetics describes how the initial rate of many single-substrate enzymes rises hyperbolically with substrate concentration and levels off at Vmax. Km is the [S] at which the rate is ½Vmax, and a lower Km usually means ti...

Receptor tyrosine kinases (RTKs) are membrane receptors whose intracellular domain is an enzyme that phosphorylates tyrosine residues. Ligand binding (growth factors, insulin) typically causes dimerization and autophosphorylation, which...

Base pairing is the hydrogen bonding between complementary nitrogenous bases: A with T (or U in RNA) by two H-bonds, and G with C by three. Because G–C pairs are stronger, DNA with more G–C content melts at a higher temperature.

A hydrolase is an enzyme that cleaves a bond by adding water across it; proteases, lipases, nucleases, phosphatases and amylase are examples. Testable point: hydrolases use water, while lyases cleave without water or oxidation and ligase...

An organic molecule is a carbon-based compound, typically containing C–H bonds, with carbon bonded to H, O, N, S, or P. Biological macromolecules (carbohydrates, lipids, proteins, nucleic acids) are organic, and their behavior depends la...

Bioenergetics is the study of how cells transform and transfer energy, using thermodynamics and free energy (ΔG). Most testable: cells couple exergonic reactions, such as ATP hydrolysis, to endergonic ones so the overall ΔG is negative.

A side chain (R group) is the variable group on an amino acid's α carbon. It sets the amino acid's size, polarity, and charge. Testable point: side chains, not the repeating backbone, drive tertiary structure, with hydrophobic ones usual...

Glyceraldehyde is the three-carbon aldose (an aldotriose), HOCH₂–CHOH–CHO, with one stereocenter. It is the reference for D/L sugar nomenclature, and its phosphate, G3P, is a key glycolysis intermediate. D/L is structural, not the sign o...

Telomere shortening is the loss of repeat DNA from the ends of linear chromosomes with each round of replication, because DNA polymerase cannot fully copy the lagging-strand end. Most somatic cells have little telomerase, so critically s...

Enzyme inhibition is a decrease in an enzyme's reaction rate caused by an inhibitor, which can bind reversibly or irreversibly. Most testable: a competitive inhibitor raises apparent Km with Vmax unchanged, while a pure noncompetitive in...

Protein folding is the process by which a polypeptide chain adopts its functional three-dimensional (native) shape. The primary sequence largely determines the fold, and the hydrophobic effect is a major driving force. Denaturation unfol...

Cooperative binding means binding of a ligand to one subunit of a multisubunit protein changes the affinity of the other subunits for that ligand. In hemoglobin, O₂ binding raises affinity at the remaining sites, giving a sigmoidal (S-sh...

DHAP (dihydroxyacetone phosphate) is a three-carbon glycolytic intermediate made when aldolase splits fructose 1,6-bisphosphate. Triose phosphate isomerase converts it to glyceraldehyde 3-phosphate, so both halves of glucose go through t...

Fluidity is how easily lipids and proteins move sideways within the membrane bilayer. Higher temperature, shorter chains and more cis double bonds (unsaturation) increase it. Cholesterol is a buffer: it lowers fluidity when warm and keep...

The enzyme-substrate complex (ES) is the intermediate formed when a substrate binds noncovalently at an enzyme's active site, before product forms. Most testable: at high [S] nearly all enzyme is in ES form, so the rate levels off at Vmax.

Orexin (hypocretin) is a neuropeptide made in the lateral hypothalamus that promotes wakefulness and stimulates appetite, acting through G protein-coupled receptors. Most testable: losing orexin neurons or signaling causes narcolepsy-lik...

Gene expression is the use of a gene's DNA sequence to make a functional product, usually a protein, by transcription then translation (some genes give a functional RNA only). Most testable: regulation is often greatest at transcription...

Noncompetitive inhibition: an inhibitor binds an allosteric site on the free enzyme or the enzyme–substrate complex, so less enzyme is active. In the pure case Vmax decreases and Km stays the same, and extra substrate cannot reverse it....

DNA is a double helix of two antiparallel strands, each a sugar-phosphate backbone carrying the bases A, T, G and C. A pairs with T and G with C by hydrogen bonds, so the strands are complementary and, in double-stranded DNA, A = T and G...

Glycogen synthesis (glycogenesis) builds the branched glucose-storage polymer glycogen, mainly in liver and muscle, using UDP-glucose as the activated donor. Glycogen synthase adds glucose by α-1,4 bonds, and insulin promotes the process...