programmed cell death A form of cell death in which a cell kills itself by activating an intracellular death program.
prokaryote   Single-celled microorganism whose cells lack a well-defined, membrane-enclosed nucleus. Either a bacterium or an archaeon. (Figure 1–17) promoter Nucleotide sequence in DNA to which RNA polymerase binds to begin transcription. (Figure 7–17) proteasome   Large protein complex in the cytosol with proteolytic activity that is responsible for degrading proteins that have been marked for destruction by ubiquitylation or by some other means. (Figures 6–83 and 6–84) protein   The major macromolecular constituent of cells. A linear polymer of amino acids linked together by peptide bonds in a specific sequence. (Figure 3–1) protein activity control The selective activation, inactivation, degradation, or compartmentalization of specific proteins after they have been made. One of the means by which a cell controls which proteins are active at a given time or location in the cell.
protein glycosylation   Process of transferring a single saccharide or preformed precursor oligosaccharide to proteins.
protein kinase Enzyme that transfers the terminal phosphate group of ATP to one or more specific amino acids (serine, threonine, or tyrosine) of a target protein.
protein kinase C (PKC) Ca2+-dependent protein kinase that, when activated by diacylglycerol and an increase in the concentration of cytosolic Ca2+, phosphorylates target proteins on specific serine and threonine residues. (Figure 15–29) protein phosphatase Enzyme that catalyzes phosphate removal from amino acids of a target protein.
protein subunit   An individual protein chain in a protein composed of more than one chain.
protein translocation   Process of moving a protein across a membrane.
protein translocator   Membrane-bound protein that mediates the transport of another protein across a membrane.  (Figure 12–21) protein tyrosine phosphatase   Enzyme that removes phosphate groups from phosphorylated tyrosine residues on proteins.
proteoglycan   Molecule consisting of one or more glycosaminoglycan chains attached to a core protein.  (Figure 19–38) proteomics   Study of all the proteins, including all the covalently modified forms of each, produced by a cell, tissue, or organism. Proteomics often investigates changes in this larger set of proteins—in “the proteome”—caused by changes in the environment or by extracellular signals.
proto-oncogene   Normal gene, usually concerned with the regulation of cell proliferation, that can be converted into a cancer-promoting oncogene by mutation.
protofilament   Linear string of microtubule subunits joined end to end; multiple protofilaments associate with one another laterally to construct and provide strength and adaptability to microtubules.
proton (H+) Positively charged subatomic particle that forms part of an atomic nucleus. Hydrogen has a nucleus composed of a single proton (H+).
proton-motive force   The force exerted by the electrochemical proton gradient that moves protons across a membrane.
protozoan parasite Parasitic, nonphotosynthetic, single-celled, motile eukaryotic organism, for example Plasmodium.
pseudogene Nucleotide sequence of DNA that has accumulated multiple mutations that have rendered an ancestral gene inactive and nonfunctional.
purified cell-free system Fractionated cell homogenate that retains a particular biological function of the intact cell, and in which biochemical reactions and cell processes can be more easily studied.
purifying selection Natural selection operating in a population to slow genome changes and reduce divergence by eliminating individuals carrying deleterious mutations.