Q
(1) See
temperature coefficient.
(2)
queuosine.
(3) glutamine (see amino acids).
Q-band (in ESR)
See electron
spin resonance.
Q bases
Bases
formed by the modification of guanine in tRNA; they contain a pentenyl ring
attached (via NH) to the methylngroup of N-methylguanine. An example is
queuine, the Q base of Queuosine.
Q-cycle(protonmotive Q-cycle)
A hypothetical pathway originally proposed (as part of the
classical respiratory loop model) to account e.g. For proton extrusion
at complex iii of the mitochondrial electron transport chain; this
pathway requires the presence of oxidized and reduced forms of
ubiquinone (UQ and UQH2, respectively) and the free radical, semiquinone
(UQH). In one form of the Q-cycle, an electron which enters the matrix
side of Complex III is taken up, together with one proton from the
matrix side, by UQH – forming UQH2. On passing to the cytoplasmic side, UQH2 undergoes
oxidation to UQH – one proton being extruded, and one electron passing back to
the matrix side via b-type cytochromes; this electron, together with a
proton from the matrix side, reduces UQ, thus regenerating UQH at the matrix
side. At the cytoplasmic side, the UQH formed from UQH2 undergoes oxidation to
regenerate UQ, one proton being extruded, and one electron passing to cyt c1.
Q enzyme
A branching enzyme which can convert amylose into an amylopectin-type
polysaccharide; it cannot introduce branches into amylopectin. It occurs in
certain algae and in higher plants.
Q fever
In man, an acute disease caused by Coxiella burnetii. Infection
may occur e.g. by the inhalation of contaminated dust or the ingestion
of contaminated milk. After an incubation period of 2–3 weeks there is a
sudden onset with headache, malaise, fever, muscular pain, and (often)
respiratory symptoms (pneumonitis); there is no rash. Complications
(e.g. endocarditis) may occur but the disease is rarely fatal. Diagnosis
may include e.g. serological tests and/or attempts to culture C.
burnetii from blood or sputum samples. Tetracyclines and chloramphenicol
have been used therapeutically. Reservoirs of infection occur e.g. in
sheep and cattle and in argosid and ixodid ticks.
Quantitative character
A heritable feature in a population that varies continuously as a result
of environmental influences and the additive effect of two or more genes (polygenic inheritance).
Quaternary structure
The particular shape of a complex, aggregate protein, defined by the
characteristic three-dimensional arrangement of its constituent subunits, each
a polypeptide.
Queen
In social insects (ants, termites, and some species of bees and wasps),
the fertile, or fully developed, female whose function is to lay eggs.
Quiescent
center
A region
located within the zone of cell division in plant roots, containing
meristematic cells that divide very slowly.