Keto acid  Molecule formed from amino acid metabolism and containing carbonyl (-CO-) and carboxyl (-COOH) groups.

Ketone (= Ketone body)  Product of fatty acid metabolism that accumulates in blood during starvation and in untreated diabetes mellitus. e.g. acetoacetic acid, acetone, or b-hydroxybutyric acid

Kinase  Enzyme that transfers a phosphate (usually from ATP) to another molecule.

Klenow fragment (of DNA polymerase I) A DNA ploymerase enzyme, obtained by chemical modification of E.coli DNA polymerase I, used primarily in chain termination DNA sequencing.

Km  A parameter that describes the affinity of an enzyme for its substrate and equals the substrate concentration that yields the half-maximal reaction rate; also called the Michaelis constant. A similar parameter describes the affinity of a transport protein for the transported molecule or the affinity of a receptor for its ligand, since the kinetics of transport and ligand binding are similar to those of enzymatic reactions.

Knockout animal  An animal whose endogenous gene for a particular protein has been deleted or mutated to be non-functional.

Krebs cycle  Mitochondrial metabolic pathway that utilizes fragments derived from carbohydrate, protein, and fat breakdown and produces carbon dioxide, hydrogen, and small amounts of ATP. This cycle is called tricarboxylic acid (TCA) cycle, citric acid cycle.

Kupffer cells  Phagocytic cells which line the liver sinusoids.