tein kinase phosphorylates a protein, using ATP, and
the phosphorylation modulates the shape and activity
ofproteins. Protein kinase A and protein kinase C are
involved in intracellular signal transduction in most
human cells.
See alsoPROTEIN PHOSPHATASE.
protein machines Genomes are “brought to life” by
being read out or “expressed” according to a complex
set of directions embedded in the DNA sequence. The
products of expression are proteins that do essentially
all the work of the cell: they build cellular structures,
digest nutrients, execute other metabolic functions, and
mediate much of the information flow within a cell and
among cellular communities. To accomplish these
tasks, proteins typically work together with other pro-
teins or nucleic acids as multicomponent “molecular
machines,” structures that fit together and function in
highly specific, lock-and-key ways.
protein phosphatase (phosphoprotein phosphatase)
An enzyme that removes a phosphate group from a
protein by the use of hydrolysis; opposite effect of a
PROTEIN KINASE.
proteoglycan Atype of glycoprotein with high car-
bohydrate content; component in the extracellular
matrix of animal cells. Composed of one or more gly-
cosaminoglycans, long polysaccharide chains covalent-
ly linked to protein cores.
protoderm One of the three primary meristem tis-
sues differentiated from the apical meristem; the outer-
most tissue. The protoderm differentiates into the
plant’s epidermis of the roots and shoots.
proton An atomic particle found in the nuclei of
atoms along with a neutron, but the proton has a posi-
tive electric charge.
protonephridium An example of primitive kidneys for
osmoregulation and excretion among platyhelminthes.
Protonephridium consists of a network of two or more
closed longitudinal branched tubules running the length
ofthe body. The tubular system drains into excretory
ducts that empty into the environment through nephrid-
iopores, small external openings.proton motive force Energy or force created by the
transfer of protons (hydrogen ions) on one side only of a
cell membrane and across the membrane during chemios-
mosis; an electrochemical gradient that has potential
energy. This force can be channeled to operate rotating
flagella, generate ATP, and other needed activities.proton pump Proton pumps are a type of active
transport that use the energy of ATP hydrolysis to force
the transport of protons out of the cell, thus creating a
membrane potential.proto-oncogene A normal gene that can become an
active oncogene, one that is capable of causing cells to
change into cancer cells, by mutation or insertion of
viral DNA.protoplasm The living material within cells.protoplast Any cell from which the outer cell wall
or membrane has been removed, leaving only the cell
contents.protoporphyrin IX The PORPHYRIN LIGANDof HEME
b. Heme b is a Fe(II) porphyrin complex readily isolated
from the hemoglobin of beef blood, but it is also found
in other proteins, including other HEMOGLOBINs, MYO-
GLOBINs, CYTOCHROME P-450, CATALASEs, PEROXIDAS-
Es, as well as b type CYTOCHROMEs. Protoporphyrin IX
contains four methyl groups in positions 2, 7, 12, and
18, two vinyl groups in positions 3 and 8, and two pro-
pionic acid groups in positions 13 and 17.protostome Any of the group of coelomates, such as
annelids, mollusks, and arthropods, whose embryonic278 protein machines