NST103_Part4_Lecture5_ROS_VitaminC_and_E

what are Reactive oxygen species ROS

• hydroxide radical>> superoxide >> hydrogen peroxide

• formed by 1e- transfer reaction from water redox state to oxygen redox state

• damage DNA, enzymes, and membranes

• used to kill microbes by neutrophils

hydroxyl radical

formed from Fe2+ and H2O2 during fenton reaction

• why cells try to keep low Fe2+ and H2O2

• Fe2+ + H2O2 —> Fe3+ + OH- + OH•

very reactive!

superoxide

O2•– can form upon 1e reduction of O2 by FADH2 or Fe2+

reacts with certain enzymes selectively

• FeS cluster susceptible to damage by O2• and produce dangerous Fe2+ as product

inactivation of FeS cluster containing enzymes is why E. coli/yeast cannot grow at high [O2]

hydrogen peroxide

formed from many sources (enzymes, O2•- detoxification

least reactive ROS

fenton reaction

ROS defense systems in cells

superoxide dismutase (SOD)*

catalase (Cat)*

glutathione peroxidase (GPX)*

peroxiredoxin (PRDX)*

vitamin C, E

vitamin C deficiency

scurvy

bleeding under skin, joint pain, corkscrew hair growth

caused by disfunction of collagen hydroxylases in absence of vitamin C

vitamin C synthesis ( animals vs humans )

can be synthesized by mammals

humans are missing only 1 enzyme for vitamin C biosynthesis: L-gulonolactone oxidase (GULO)

• also absent in other primates that eat vitamin C rich fruits

vitamin C transport across cell membranes

ascorbic acid is absorbed through sodium-coupled transporter SVCT1/2

dehydroascrobic acid (DHA) is absorbed through diffusion driven glucose transporters (GLUT1/3)

• DHA reduced to ascorbic acid inside cells

vitamin C absorption

absorption is saturable due to limited expression of transporters in enterocytes

taking more vitamin C does not increase blood Vit C after a certain threshold

vitamin C distribution across tissus

levels are 10x higher inside cells compared to blood, driven by sodium-coupled intake

concentration varies across tissues

vitamin C functions

ROS scavenger

required in catalytic amounts for repair of iron oxygenases

• collagen biosynthesis

• DNA demethylation

• histone demethylation

co substrate for several copper oxygenase enzymes

conversion of Fe3+ to Fe2+ to facilitate Fe transport across membranes

vitamin C mononuclear iron oxygenases

use oxygen to hydroxylate substrates (add hydroxyl group)

enzyme contains Fe coordinated with His, Asp, and a-KG

rxn mech similar to CyP450 enzymes, but Fe bound to heme in Cyp450 instead of directly coordinated with other res (his, Asp)

ascorbate needed to reduce Fe3+ —> Fe2+

vitamin C reactivation of enzymes stuck in FeIV=O state

mononuclear iron oxygenases can get stuck in FeIV=O state without a substrate present

ascorbate serves as substrate to recycle enzyme back to Fe2+ active state

collagen hydroxylases and vitamin C

vit C required to reactivate enzyme that hydroxylates proline and lysine res on collagen

scurvy caused by disfunction of collagen hydroxylases in absence of vitamin C

function of collagen hydroxylase

collagen: most abundant animal protein, main protein in connecitve tissue

proline is second most abundant AA in collagen, and most prolines are hydroxylated

hydroxylated lys allow for attachment of oligosaccharides

Pro and Lys hydroxylation essential for producing functional collagen chains

HIF1α

hypoxia inducible factor 1 alpha

transcription factor whose levels are regulated by [O2]

activates transcription of genes required for survival under low [O2]

• glycolytic genes for fermentative ATP prodiction

• EPO for increased RBC production

histone demethylases

epigenetic mark that controls gene expression

2 types of enzymes catalyze

• KDM1A/LSD1: FAD dependent

• JHDM: Fe2+, O2, α-KG dependent, require catalytic amounts of vitamin C

DNA demethylases

DNA methylation at promoter elements for epigenetic modification

TET: DNA demthylase that Fe2+, O2, α-KG dependent, require catalytic amounts of vitamin C

ROS defense and vitamin C

ascorbate can react with radicals and quench radical chain reactions

ascorbate radical is stable and doesn’t spontaneously react with other molecules

DHA reduced back to ascorbate by NADPH-utilizing glutathione reductase

vitamin C is water soluble so it can react with water soluble radicals

radical chain reactions in polyunsaturated lipids (and vit E)

polyunsaturated lipids can react with hydroxyl radical and cause chain rxn that damage lipid molecules

vit E intercepts ROO• radical to stop chain rxn

poly- and mono-unsaturated lipids are more susceptible to chain reactions because it is easier to subtract e from unsaturated carbon

vitamin E and ROS defense

vit E can react with radicals to form a stable radical that stops chain reactions

• stable due to possibility of resonance states above where electron density of radical can be delocalized

Vit E is lipophillic, so it reacts with lipid radicals unlike vit C

vitamin E transport

lipophilic: transported across tissue with other lipids in LDL and CM

vit E protects lipids in LDL from oxidation as there is high [O2] in blood which promotes oxidation (O2 more soluble in lipids than water)

vitamin E tissue distribution

most of body Vit E is accumulated in adipose tissue where it helps to minimize stored lipid oxidation