Vitamin C Flashcards
Vitamins are often required as
enzyme cofactors
Ascorbate is an effective
reducing agent
reduces ROS and transition metal ions
acts as a chaperone to prevent over oxidation of Fe in deoxygenase enzymes
ROS are produced by
partial reduction of oxygen during metabolism
e.g. fenton reaction
(from oxidase enz and e transport chain in mitochondria)
What do ROS do
very reactive and cause oxidative damage to protein, lipids and DNA
Ascorbate reacts with
H202 and other free radicals produced by the interaction of ROS with organic compounds
produces monodehydroascorbate radical (MDHA.)
.H + .R -> MDHA. + RH
What happens to the unpaired electron in MDHA?
it is delocalised into the ring making it more stable and less reactive
the MDHA radical is recycled back to ascorbate in 2 ways:
MDHA reductase
the ascorbate glutathione cycle
mdha reductase pathway
1 step
direct route
NADPH donates H (becoming NADP) to form ascorbate
mdha reductase catalyses
the ascorbate glutathione pathway
2 step
DHA (dehydroascorbate) production followed by DHA reduction
2 molecules of mdha readily disproportionate to form ascorbate and DHA:
second step catalysed by DHA reductase
2MDHA. –> DHA + asc
DHA –> Ascorbate (reduced by thiols GSH -> GSSG)
Oxidised GSSG from the asc-glut pathway is regenerate to GSH by
NADPH and catalysed by
glutathione reductase
Where are the acidic and reducing properties located on ascorbate?
At the ene-diol group on C2 and C4
In healthy cells: 90% ascorbate, 10% DHA, trace MDHA
DHA is the oxidised form of
ascorbate
As well as reducing DHA, GSH (glutathione) removes
H202, using enzyme GPX (glutathione peroxidase)
forming GSSG
GSH is a central player in thiol based antioxidant/redox signalling systems
GSSG is
2 GSH connected by a disulfide bridge
GSSG is regenerated using
NADPH by enzyme GR (glutathione reductase)
use of NADPH links antioxidant system to central metabolism
