gallic acid:
a polyphenolic organic acid found in many plants usually bound
to some organic alcohol such as a glycoside or a bioflavonoid
forming an ester. The structure of gallic acid resembles benzoic acid
with three hydroxyl groups bound to the aromatic ring at the
opposite side of the carboxyl group. Due to their phenolic structure,
gallates are effective oxyradical quenchers. Upon oxidation
gallates become bitter tasting and astringent orthoquinones.
glucose-6-phosphate (G6P):
an ester of phosphoric acid with the hydroxyl group at carbon 6
of glucose; a metabolically activated form of glucose produced
by enzymatic phosphorylation from ATP. This is the first step
in the utilization of glucose in anaerobic glycolysis and in
the hexose monophosphate shunt.
glucose-6-phosphate dehydrogenase (G6PDH):
an enzyme which reduces NADP+ using hydrogen abstracted from G6P;
the first step in the hexose monophosphate shunt which produces
large quantities of NADPH.
glutamic acid (glu):
an amino acid which has a side chain composed of three methylene
groups with a terminal carboxyl group.
glutaredoxin (Grx):
an enzyme possessing two redox active cysteine residues which
can activate ribonucleoside reductase (RR). Grx can accept hydrogen
atoms from glutathione (GSH) or from protein disulfide isomerase
(PDI) and subsequently transfer them to RR.
glutathione (GSH):
gamma-glutamyl-cysteinyl-glycine; a ubiquitous tripeptide with
a redox active thiol group. GSH has numerous roles as a physiologic
hydrogen carrier and antioxidant. Many thiol compounds including
certain enzymes, cofactors, and transcription factors are maintained
in the reduced state by the hydrogen donating function of GSH.
An important selenium containing peroxidase depends on GSH for
activation.
glutathione disulfide (GSSG):
a dimer of glutathione produced by oxidative coupling of GSH.
Upon donation of its hydrogen atom to an oxidant, GSH becomes
the glutathyil radical (GS*). Two of these couple to become GSSG.
GSSG can accept hydrogen from numerous donors thus behaving as a weak
oxidant. GSSG also participates in thiol-disulfide exchange reactions.
Intracellular GSSG triggers the release of NF-kappa-B in certain cells.
glutathione peroxidase (GP):
glutathione:peroxide oxidoreductase; an enzyme which oxidizes
glutathione and reduces peroxides. GP picks up hydrogen atoms
from GSH and passes them on to hydrogen peroxide or to lipid
peroxide(s) producing water and / or alcohols. GP utilizes
the amino acid selenocysteine in its redox active center.
glutathione reductase (GR):
NADPH:glutatione disulfide oxidoreductase; a flavoprotein
(utilizing FAD) which transfers electrons from NADPH to GSSG
producing NADP+ and GSH. GR maintains most of the glutathione
of the cytosol in the reduced state. GR activity is increased
whenever oxidants deplete hydrogen atoms from the GSH pool.
The production of more GP is induced in response to stimulation
by most oxidants including oxyradicals. When GR activity is high
much NADPH is converted to NADP+, thus less reducing equivalents
are available for other functiuons such as syntheses.
glutathione-S-transferase (GST):
a conjugase which covalently attaches GSH to various xenobiotics
or electrophiles. The process sacrifices one molecule of GSH
for each molecule of xenobiotic so conjugated. While the glutamate
and glycine portions are recovered, the cysteine is lost permanently.
Excessive activity of GST can cause cysteine and/or glutathione
deficiency, oxidative stress, and immune dysregulation.
glycerol-3-phosphate (G3P or H2COH-HCOH-H2C-OPO3--):
an ester of phosphoric acid and glycerol. This compound is used
as a shuttle to transport hydrogen atoms into the mitochondria.
It is also an important precursor to numerous phosphatides.
glycerol-3-phosphate dehydrogenase (GPD):
the flavoprotein at the mitochondrial membrane which accepts
two hydrogen atoms from G3P and transfers them to ubiquinone.
GPD activity is upregulated by thyroid hormones and by DHEA.
Its function is deficient in many cancers.
glycerol phosphate shuttle:
the transfer sequence which carries reducing equivalents from
cytoplasmic NADH to intramitochondrial ubiquinone using G3P
as one of the carriers. The complete sequence is:
NAD+/NADH...DHAP/G3P...FAD/FADH2...CoQ/CoQH2...cytochromes...O2/H2O
glycine (gly):
the simplest amino acid having no side chain; part of GSH.
glycolysis:
a multistep pathway of glucose metabolism whereby glucose is
broken down and eventually converted to pyruvic acid. A small
amount of ATP is generated by this process. Under anaerobic
conditions puruvate is utilized as the final hydrogen acceptor
instead of oxygen thus lactic acid is produced.
glycoside:
one of many compounds altered by covalent attachment to a sugar
molecule. The process renders the original compound more water
soluble and usually less chemically active.
glycosylated protein(s):
the addition product of monoglyceride to protein. The aldehyde
groups of simple sugars in the open configuration can react with
the amino groups of numerous proteins to produce imines.
While this initial reaction is reversible, subsequent reactions
of these imines render the adduct permanent and the affected
protein nonfunctional. This problem is prown to occur under
conditions of hyperglycemia and accounts for certain
degenerative effects of aging and of diabetes.
glyoxal (CHO-CHO):
the simplest dialdehyde composed of two covalently bound aldehyde
groups. The two carbonyl groups can conjugate and redox cycle.
Nucleophiles such as thiols and primary amines readily add to glyoxal.
glyoxalase:
an enzyme requiring reduced glutathione (GSH) as cofactor which
serves to convert alpha-keto-aldehydes to alpha-hydroxy-carboxylic
acids. The process occurs in three steps: 1) nonenzymatic addition of
GSH to the aldehyde; 2) glyoxylase I shifts two hydrogen atoms from the
number one to the number two carbon position; 3) glyoxalase II
hydrolyses the intermediate releasing GSH and the alpha-hydroxy-acid.
guanidine (NH2-CNH-NH2):
a highly alkaline amine composed of 2 amino groups and 1 imino group
attached to a central carbon atom. Guanidine is a product of the
putrifaction of arginine. When present in high levels as in uremia
it mildly inhibits mitochondrial function, thus it is a respiratory
poison. Other compounds which contain the guanidino group include:
methyl guanidine, agmatine, creatine, arginine, streptomycin,
cimetidine, guanfacine, aminoguanidine, and metformin.
