1,800+ MCAT terms, defined.

An enzyme-linked receptor is a membrane receptor whose intracellular domain is an enzyme or is bound to one. The classic example is the receptor tyrosine kinase, which autophosphorylates tyrosines after ligand binding and starts cascades...

A Haworth projection is a flat drawing of a cyclic sugar, with the ring viewed edge-on and each substituent pointing up or down. For D-sugars, groups on the right in the Fischer projection point down, and the α anomer has its anomeric OH...

In noncompetitive inhibition, an inhibitor binds an allosteric site (not the active site) and lowers the amount of working enzyme. In the pure case Vmax decreases, Km is unchanged, and adding more substrate cannot reverse it.

Kinesins are motor proteins that use ATP hydrolysis to walk along microtubules, carrying vesicles and organelles. Most kinesins move toward the plus end, usually toward the cell periphery (anterograde transport in axons), while dynein mo...

A DNA library is a collection of DNA fragments cloned into vectors and held in host cells. A genomic library covers the whole genome, including introns; a cDNA library is made from mRNA, so it has only expressed sequences and no introns....

Sialic acid is a family of nine-carbon acidic sugars, usually found at the ends of glycan chains on cell-surface glycoproteins and glycolipids. Its carboxylate is negatively charged at physiological pH, and viral neuraminidase cleaves it.

Succinyl CoA synthetase is the citric acid cycle enzyme that converts succinyl CoA to succinate and uses the released thioester energy to make GTP (or ATP, depending on the isoform) from GDP (or ADP) and Pi. It is the cycle's substrate-l...

Cytosine deamination is the hydrolytic loss of cytosine's amino group, which converts it to uracil. In DNA, uracil is removed by base excision repair; if it persists through replication, it pairs with adenine and a G·C pair becomes A·T (...

The inner mitochondrial membrane is the folded inner membrane (cristae) that holds the electron transport chain and ATP synthase. It is largely impermeable to H⁺, so the chain's proton pumping builds a gradient across it. The testable po...

Pancreatic means "of the pancreas." It is a modifier, not a single concept. In biochemistry it usually points to pancreatic digestive enzymes (amylase, lipase, trypsin and other proteases), secreted mostly as inactive zymogens into the s...

Sanger sequencing reads a DNA sequence by copying a template with polymerase in the presence of dideoxynucleotides (ddNTPs), which lack a 3′-OH and stop chain growth. The fragments are separated by size, and the gel is read from smallest...

Taq polymerase is a heat-stable DNA polymerase from the hot-spring bacterium Thermus aquaticus. It survives the roughly 95 °C denaturation step of PCR, so it does not need to be re-added each cycle. The testable point is that PCR uses it...

A glycosidic bond joins a sugar's anomeric carbon to an oxygen (or, in nucleosides, a nitrogen) of another molecule, formed by condensation. Most testable: α versus β configuration decides enzyme specificity, so humans digest α-1,4 starc...

A deoxy sugar is a carbohydrate in which a hydroxyl group has been replaced by a hydrogen. The standard example is 2-deoxyribose in DNA, which lacks the OH on C2. The most testable point: the missing 2′-OH makes DNA's backbone more resis...

A disaccharide is two monosaccharides joined by a glycosidic bond, formed by dehydration (loss of H₂O). Maltose, lactose and sucrose are the standard examples. Most testable: a disaccharide is reducing when an anomeric carbon is still fr...

The lac repressor is the E. coli protein that binds the lac operator and blocks transcription of the lactose-use genes. Allolactose, made from lactose, binds it and releases it from DNA, so the operon is inducible. Mutants that cannot bi...

Non-superimposable means one structure cannot be laid over another so every atom matches, even after rotating it. A chiral molecule and its mirror image are non-superimposable and are called enantiomers, like D- and L-amino acids. Diaste...

A carboxyl group is –COOH: a carbon double-bonded to one O and single-bonded to an OH. In amino acids it is the acidic group at the C-terminus and in Asp and Glu side chains. Its pKa is well below 7.4, so at physiological pH it is deprot...

A thioester is an acyl group bonded to sulfur, R–C(=O)–S–R′, as in acetyl-CoA and succinyl-CoA. Its hydrolysis is strongly exergonic (about −31 kJ/mol for acetyl-CoA, similar to ATP), so it acts as an activated acyl carrier that can driv...

Pyruvate decarboxylation is the step in which the pyruvate dehydrogenase complex converts pyruvate to acetyl-CoA in the mitochondrial matrix, releasing CO₂ and reducing NAD⁺ to NADH. It links glycolysis to the citric acid cycle and is es...

Uncouplers are molecules or proteins that let protons leak back across the inner mitochondrial membrane without passing through ATP synthase. The gradient's energy is released as heat instead of making ATP, so electron transport and O₂ u...

A chiral carbon (stereocenter) is an sp³ carbon bonded to four different groups, so it has a nonsuperimposable mirror image. Most testable: every standard amino acid except glycine has a chiral α-carbon, and n chiral centers give at most...

Regulation by covalent modification changes an enzyme's activity by attaching or removing a chemical group, most often a phosphate on a serine, threonine or tyrosine side chain. Phosphorylation can either activate or inhibit, depending o...

Fructose 1,6-bisphosphate is the six-carbon glycolytic intermediate formed when PFK-1 uses ATP to phosphorylate fructose 6-phosphate, the committed step. Aldolase then splits it into DHAP and glyceraldehyde 3-phosphate. Gluconeogenesis b...

Isoelectric focusing separates proteins in a gel with a pH gradient and an electric field. Each protein migrates until the local pH equals its pI, where its net charge is zero and it stops. It separates by pI (charge), not by size.

Allosteric describes regulation in which a molecule (effector) binds a site on a protein other than the active site and changes the protein's shape and activity. Effectors can be activators or inhibitors, and allosteric enzymes typically...

Guanidinium is the protonated guanidino group on arginine's side chain, with its positive charge shared by resonance over three nitrogens. Its pKa is about 12.5, so it is positively charged at physiological pH, which makes arginine a bas...

The citrate shuttle moves acetyl-CoA from the mitochondrial matrix to the cytosol as citrate, because acetyl-CoA itself cannot cross the inner membrane. Cytosolic ATP-citrate lyase regenerates acetyl-CoA for fatty acid synthesis, so the...

An exergonic reaction has a negative ΔG, so it is thermodynamically spontaneous and releases free energy available to do work. The testable point: ΔG < 0 says nothing about speed, which depends on activation energy and catalysts.

A flavoprotein is a protein with a tightly bound flavin cofactor, FAD or FMN (made from riboflavin, vitamin B2), that takes part in redox reactions. Most testable: FAD is reduced to FADH₂, as in succinate dehydrogenase, and FADH₂ yields...

NADP⁺ is the oxidized form of nicotinamide adenine dinucleotide phosphate, an electron carrier that accepts a hydride to become NADPH. NADPH is the reducing agent for biosynthesis, and it is made mainly by the pentose phosphate pathway (...

A symport (cotransport) is a membrane transporter that moves two different substances across the membrane in the same direction at the same time. It is usually secondary active transport: the downhill movement of one solute, often Na⁺, d...