19. The molecular sources of oxidative stress, signal transduction pathways induced by oxidative stress, molecular tools and protection against oxidative stress

what is oxidative stress?

oxidative stress is a phenomenon caused by an imbalance between production/accumulation of reactive oxygen species and the ability of the tissues to detoxify these reactive products

Reactive oxygen species?

free radicals contain unpaired, reactive free electrons

the free radicals are very reactive and are able to damage several biomolecules

some notable ROS are:

what is a radical?

a molecule/atom w. an unpaired valence electron

highly reactive

sources of reactive species?

the source of ROS and RNS may be endgenous or exogenous

examples of exogenous sources are exposure to pollutants, heavy metals, certain drugs, smoking and alcohol

the endogenous ROS are produced by both physiologically important enzymes and by non-enzymatic reactions

most notably, O2⁻⋅ can be formed in the reactions of the complex I and III in the respiratory chain. semiquinone (QH) is a radical and can easily pass an electron to O2 forming O2⁻⋅. so in this case the ROS is a by-product of the respiratory chain

another enzyme forming a radical is NO-synthase. this enzyme is present in macrophages, neurons and is endothelium. NOS catalyzes arginine to citrulline reaction by releasing nitric oxide

oxidative damage to proteins?

aggregation, fragmentation, cleavage

reaction w. heme metal ion

modification of functional groups

→

change in ezymatic activity and ion transport

proteloysis

oxidative damage to DNA?

disintegration of the sugar ring

base modification

strand breaks

→

mutation

failure during translation

inhibition of protein synthesis

oxidative damage to lipids?

saturation of unsaturated bonds

preparation of reactive metabolites

→

changes in membrane fluidity and permeability

the membrane proteins are involved

pathways induced by oxidative stress?

ROS can act. the NRF2 pathway. NRF2 is a TF which regulates expression of many genes related to protection of oxidative stress. in normal conditions NRF2 is bound to Keap1, which inhibits NRF2. Keap1 targets NRF2 for ubiquination and degradation

when there is oxidative stress, Keap1 will not inhibit NRF2 and it translocates to the nucleus. in the nucleus it forms heterodimers with maf proteins and binds to the ARE

the mechanism of release from Keap1 is by oxidation of Cys residues. upon binding of ARE it induces expression of oxidative stress protection, like gene for NADPH synthesis glutathione synthesis, or other genes for elimination of ROS

PARP?

poly-ADP-ribose polymerase is a protein involved in DNA repair in physiological conditions

in case of severe DNA damage, a lot of PAR is produced.

this will deplete the cell of both NAD+, which is the source of ADP-ribose and ATP

less NAD+ = less glucose oxidation

cellular death is the result of PARP overactivation

ASK-1?

Apoptosis signal-regulating 1 is a MAPKKK (mitogen activated protein kinase kinase kinase)

when there is no oxidative stress it is bound to TRX and is an oxidative stress sensor

oxidative stress releases Trx and TRAF (TNF-⍺ receptor associated factor 2) binds and ASK-1 is activated

ASK-1 phosphorylate other MAPKK which phosphorylate JNK and p38 which leads to apoptosis

NF-kβ?

ROS can also activate NF-kβ pathway

ROS activates IKKβ activates NF-kβ

NF-kβ will induce the transcription of many pro-inflammatory cytokines, like TNF⍺, IL-1, IL-6, COX and iNOS

cellular defense against oxidative stress?

antioxidants

* compounds which functions to neutralize ROS
* they can do this by accepting or donating electrons to eliminate the unpaired condition of the free radical
* in this process they either directly react w. the radical and destroy it, or become new, more stable radicals
* e.g: glutathione, vitamin A, C and E, uric acid

enzymes

* image →