A molecular ion, CH3COO-,
derived from acetic acid.
A molecular ion, CH3COCH2COO-,
derived from acetoacetic acid.
A molecular ion, CH3COPO42-,
which plays a role in the synthesis of the four-carbon
fatty acid, butyric acid.
A (Acetyl CoA)
Originally termed "active acetate," acetyl
CoA is an important metabolic intermediate, derived
from various pathways, such as glycolysis, fatty-acid
oxidation, and degradation of some amino acids. It
also represents a key intermediate in lipid biosynthesis.
An important carrier of energy in cells, ATP is formed
from adenosine diphosphate (ADP) by an oxidation reaction
in mitochondria, or by a photo reaction in plants.
Reactions in which it participates are often driven
in the direction leading to hydrolysis (reaction with
water) of ATP. The chemical energy so released may
be utilized in many ways: it can be converted to mechanical
energy (e.g., for muscular contraction), to light
energy (for bioluminescence), or to electrical energy
(in electric fish); or it may be released as heat.
ATP also participates in numerous synthetic reactions
The generic name for numerous enzymes that are specific
for the transfer of an adenylyl group (adenosine[mono]phosphor
group) from a donor molecule (usually adenosine triphosphate,
ATP) to an acceptor, such as a nucleotide, a protein,
and a sugar.
A reaction that introduces adenylyl groups into a
compound generally by the action of an enzyme, adenylyl
Breakdown of a molecule into smaller chemical entities
in the presence of oxygen.
Microorganisms that exist as single cells or in clusters
of single cells and are able to grow in the absence
Organic acids possessing one or more amino groups
and peptides are polymers of amino acids, and so the
biological function of amino acids is quantitatively
and qualitatively important. Plants are able to synthesize
all the amino acids from simple precursors. Animals,
however, are able to synthesize only certain kinds
of amino acids.
Amino acids that the animal can synthesize in sufficient
quantities for its needs are known as nonessential,
whereas those that must be supplied in diet are essential.
Technically, amino acids are obtained by the hydrolysis
of proteins, and by chemical and microbial synthesis.
A molecular ion derived from aspartic acid, an amino
A multifunction enzyme that catalyzes two steps in
the pathway from aspartate to other amino acids, such
as lysine, methionine, and isoleucine.
An acid, CH3-CH2-CH2-COOH,
that has four carbons in it.
Chemical compounds that have one or more carbonyl
groups (=CO). They include aldehydes, ketones, and
A sequence of successive activation reactions involving
either enzymes (enzyme cascades) or hormones (hormone
cascades). In blood coagulation, for example, each
enzyme activates the next until the final substrate
is reached. In glutamine metabolism, by contrast,
the activity of its key enzyme, glutamine synthetase,
is regulated by the two systems of interconvertible
enzyme cycles, which are tightly linked. Hence a "bicyclic
cascade system." The cascade system is characterized
by a series of amplifications of a weak initial stimulus,
and may be activated at more than one point along
A fluid, made by breaking open cells, that contains
most of the constituents of the cells in question
but no intact cells.
A steroid alcohol which regulates certain cell functions
and helps build cell membranes in animal fats.
An enzyme that catalyzes the oxidation of cholesterol.
Any of various nonprotein compounds that are required,
in addition to an enzyme and a substrate, for an enzymatic
reaction to proceed.
A hormone from the adrenal cortex, cortisone is derived
from cortisol. It stimulates the formation of carbohydrate
from proteins, promotes glycogen storage in the liver,
and raises the blood sugar level.
A chemical bond formed between two atoms in a molecule
by the sharing of electrons, usually in pairs, by
the bonded atoms.
A complex protein which catalyzes a specific biochemical
reaction without changing itself. Enzymes control
digestion, muscle contraction, and many other functions
A tendency to pass or ooze fluids.
Any acid derived from fats by hydrolysis.
Inhibition of the activity of an enzyme by the accumulation
of a substance produced further along a metabolic
pathway of which the enzyme is a constituent.
The decomposition of chemical substances, brought
about by enzymes, that results in the production of
simpler substances and, often, of energy. It often
refers to the anaerobic breakdown of energy-rich carbohydrates,
such as glucose, into carbon dioxide and alcohol or
to an organic acid.
An enzyme that oxidizes formatea molecular ion
(HCOO-) derived from
folic acidby removing a hydrogen atom.
A molecular ion derived from glutamic acid, an amino
An amino acid synthesized from glutamic acid. Glutamine
is an important intermediate in various metabolic
pathways because it is the amino group donor in the
formation of many biosynthetic products as well as
being a storage form of ammonia.
An enzyme that catalyzes the conversion of glutamate
to glutamine. This is the first step in the fixation
of ammonia in plants, the ATP-dependent incorporation
into the amide position of glutamine. In E. coli,
the activity of glutamine synthetase is regulated
by both feedback inhibition and the bicyclic cascade
system, which makes it extremely responsive to the
cell's nitrogen requirements.
An enzyme that catalyzes the oxidation of hydrogen
of two molecules of glutathione to form oxidized glutathione
and two molecules of H2O.
It has a UGA-encoded selenocysteine residue at its
active site. The main role of the enzyme is to protect
hemoglobin from oxidative breakdown. The oxidized
glutathione is reduced by glutathione reductase, the
system thus contributing to a reduction of peroxide
levels in the cell.
An amino acid. With the formula, NH2-CH2-COOH,
glycine is the simplest naturally occurring amino
An enzyme that catalyzes the reductive degradation
A chemical substance produced in a living organism
which controls the rate of biochemical processes.
Hormones are transported from point to point within
the organism and thus effect action at a distance
from the region in which they are made. Thus, hormones
are part of an information-transmission system for
An amino acid.
The totality of chemical reactions within a living
organism, which results in the continual turnover
(degradation and renewal) of all cell components,
and which generates the energy consumed by the organism.
Since living organisms are open systems, metabolism
consists of numerous linked reactions, which continually
strive toward a fixed position of dynamic equilibrium
without attaining it. Thus metabolism is a steady
state, rather than an equilibrium.
Any series of connected reactions occurring in a cell
or organism. Its reactants, intermediates, and products
are called metabolites. There are over 2000 known
metabolic reactions, each catalyzed by a distinct
enzyme. The types of enzymes and metabolites vary
with the identity of the organism, the cell type,
its nutritional status, and its developmental stage.
Many metabolic pathways are branched and interconnected,
and finding a metabolic pathway out of thousands of
reactions has been one of the main research agendas
regulation (metabolic control)
A system of control mechanisms that underlies the
high degree of adaptation displayed by living organisms,
and guarantees the efficient and economic operation
of living processes.
An odorless gas, CH4,
produced by the decomposition of organic matter.
Oxidation (MFO) system (Metal-Catalyzed Oxidation
A system that has at least three elements in itoxygen,
iron, and an electron donorwhich initiates the
oxidative degradation of protein.
Cancer Institute (NCI)
Established in 1937, the National Cancer Institute
began by administering a fellowship program, as well
as conducting cancer research within its intramural
program. It became officially part of the National
Institute of Health in 1944 when the NIH was still
singular. The Journal of the National Cancer Institute
published its first issue in 1940. In 1972, the status
of NCI was elevated to a bureau-level organization
in accordance with the expanded responsibilities bestowed
on it by the National Cancer Act of 1971.
Heart Institute (NHI)
The National Heart Institute was created in 1948 as
part of the National Institutes of Health. In 1969,
it was renamed the National Heart and Lung Institute
(NHLI), reflecting expansion of its functions. In
1972, NHLI obtained bureau status within NIH. This
institute was further enlarged in 1976 to become the
National Heart, Lung, and Blood Institute (NHLBI).
Institutes of Health (NIH)
The NIH, now one of the world's foremost biomedical
centers, had a humble origin. It started in 1887 as
a one-room laboratory conducting bacteriological research
within the Marine Hospital at Stapleton, Staten Island,
New York. This laboratory, called the Laboratory of
Hygiene, moved to Washington, D.C., in 1891. With
the Ransdell Act of 1930, it was renamed the National
Institute of Health. In 1938, construction began for NIH in Bethesda, Maryland, on the
45 acres of land donated by the Wilson family. In
1948, with the creation of the National Heart Institute,
the NIH officially became plural, the National Institutes
of Health, and by 1950 it had seven institutes and
one division under its wing. Since then, the NIH has
seen rapid growth, especially in its extramural program
which administers the process of awarding grants to
researchers around the world. Currently it is composed
of 27 institutes and centers.
Localized death and degeneration of tissues in a living
organism, due, e.g., to infection or injury.
An enzyme that catalyzes the conversion of acetyl
phosphate into acetyl CoA in the presence of coenzyme
Any chain of peptide comprising 10 to 20 amino-acid
residues connected by peptide linkages. Unlike a protein,
it usually lacks appreciable tertiary structure in
solution and is not liable to irreversible denaturation.
Enzymes that hydrolyze (breaks down in the presence
of water) peptide bonds in proteins and peptides.
A major macromolecular constituent of living organisms.
All enzymes are proteins. A protein is a linear polymer
of amino acids linked by peptide bonds in a specific
sequence. In the biosynthesis of the polypeptide chain,
any of twenty different amino acids may be incorporated,
according to the genetic instruction of the cell.
turnover (Protein degradation)
Like other components of living cells constantly turning
over, proteins have lifetimes that range from as short
as a few minutes to weeks or more. In any case, cells
continuously synthesize proteins from and degrade
them to amino acids. This seemingly wasteful process
has three functions: (1) to store nutrients in the
form of proteins and to break them down in times of
metabolic need; (2) to eliminate abnormal proteins
whose accumulation would be harmful to the cell; and
(3) to permit the regulation of cellular metabolism
by eliminating superfluous enzymes and regulatory
proteins. Controlling the rate of protein turnover
is therefore as important to the cellular and organismal
economy as is controlling its rate of synthesis.
Any molecular entity, charged or uncharged, that possesses
an unpaired electron (but normally excluding any paramagnetic
metal ion). A radical is usually very unstable and
thus highly reactive. Radical character is indicated
in a formula by a centered dot symbolizing the unpaired
electron. The term "free radical" is now
preferably restricted to any radical that does not
form part of a radical pair, that is, two radicals
in close proximity in liquid solution. "Radical
scavenger" refers to any substance that can readily
bind with, and thereby eliminate, radicals.
A radioactive substance that is added to a metabolic
system in quantities too small to perturb the system.
It is used to follow, by isolating the radioactive
intermediates formed, the sequence of transformations
undergone normally by the same (nonradioactive) molecule
under similar conditions.
A metallic element whose atomic number is 34. It occurs
in various colors (gray, red, black). Selenium is
an essential trace element, required for the formation
of selenoproteins, notably glutathione peroxidase.
Measurement of red-cell glutathione peroxidase activity
gives an index of selenium status. The most distinctive
clinical feature of dietary selenium deficiency is
myopathy, a disorder of muscle tissue.
An amino acid in which selenium takes the place of
sulfur in the amino acid cysteine. Its formula is
It is an essential component of selenium-containing
proteins, or selenoproteins.
A general class of substances widely found in nature,
characterized by the presence of a molecular part
called the cyclopentano-phenanthrene ring. The steroid
includes substances which are normal constituents
of animal structure, such as cholesterol, and many
other substances possessing various specific physiological
actions in the animal organism.
Any of the trinucleotide codons, UGA, UAG, and UAA,
that signal the termination of translation of a messenger
RNA molecule and the release of the nascent polypeptide
chain. A codon is a group of three consecutive nucleotide
bases (base triplet) in a given messenger RNA molecule
that specifies the synthesis of a particular amino
acid in the polypeptide chain or signals the beginning
or the end of the message.
An amino acid.
A molecular ion derived from uridylic acid. It has
a uracil as its base.
The existence of vitamin B12
came to light in 1926 when George Minot and William
Murphy discovered that pernicious anemia, an often
fatal disease of the elderly characterized by decreased
numbers of red blood cells, low hemoglobin levels,
and progressive neurological deterioration, could
be treated by the daily consumption of large amounts
of raw liver. Vitamin B12
was not isolated until 1948. This vitamin is synthesized
neither by plants nor animals but only by a few species
of bacteria. Humans obtain almost all of their vitamin
B12 directly from their
diet, particularly from meat.
Jeremy M. Berg, John L. Tymoczko, and Lubert Stryer,
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Paul Singleton and Diana Sainsbury, eds. Dictionary
of Microbiology and Molecular Biology, 2nd ed.
New York: John Wiley & Sons, 1996.
A. D. Smith, et al., eds. Oxford Dictionary of
Biochemistry and Molecular Biology. Oxford and
New York: Oxford University Press, 1997.
Friedrich W. Stöcker, Gerhard Dietrich, and
VEB E. A. Brockhaus, eds. Concise Encyclopedia
of Biology, translated and revised by Thomas
A. Scott. Berlin and New York: Walter de Gruyter &
Donald Voet, Judith G. Voet, and Charlotte W. Pratt,
Fundamentals of Biochemistry. New York: Wiley,