Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

karyopherin One of a family of nuclear transport proteins that functions as an importin, exportin, or occasionally both. Each karyopherin binds to a specific signal sequence in cargo proteins moving in or out of the nucleus.

karyotype Number, sizes, and shapes of the entire set of metaphase chromosomes of a eukaryotic cell. (Chapter 8 opening figure)

keratins A group of intermediate filament proteins found in epithelial cells that assemble into heteropolymeric filaments. (Figure 18-49)

kinase An enzyme that transfers the terminal (γ) phosphate group from ATP to a substrate. Protein kinases, which phosphorylate specific serine, threonine, or tyrosine residues, play a critical role in regulating the activity of many cellular proteins. See also phosphatases. (Figure 3-33)

kinesins A class of motor proteins that use energy released by ATP hydrolysis to move toward the (−) end of a microtubule. Kinesins can transport vesicles and organelles and play a role in chromosome movement during mitosis. (Figures 18-18, 18-19, 18-20)

kinetic energy Energy of movement, such as the motion of molecules.

kinetochore A multilayer protein structure located at or near the centromere of each mitotic chromosome from which microtubules extend toward the spindle poles of the cell; plays an active role in movement of chromosomes toward the poles during anaphase. (Figure 18-40)

K_m 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. (Figure 3-24)

knockdown, siRNA See siRNA knockdown.

knockout, gene See gene knockout.