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BHCS2020 Summary Notes
Bohr’s atomic model of oxygen
• Atomic number = 8
• 2 unpaired electrons free for bonding
• Covalently bonds with another O atom
• The unpaired electrons in the outer pi orbital have parallel spins
• If gains/losses more than 2 electrons, forms a reactive oxygen species (ROS)
• O2- (superoxide) has gained ana extra electron
• Additional electron needs an opposing spin else it is repelled
• The extra unpaired electron makes ROS highly reactive
In most cells, ATP/ADP ratio is controlled by the ATP demand
Cytosolic ATP/ADP ratio determines if the cell metabolism is predominately oxidative or glycolytic
Mitochondria – powerhouse of the cell
• Adenine nucleotide translocator (ANT)
o Exports ATP from mitochondrial matrix
o Imports ADP into mitochondrial matrix
o Most abundant protein in inner mitochondrial membrane
• Outer membrane is permeable to molecules under 500Da
• Inner membrane impermeable – need to use carriers/channels
Cellular respiration summary (page 10)
• Glycolysis in cytoplasm
o Glucose pyruvate
o Net gain of 2 ATP, 2 NADH and 2 pyruvates
• Mitochondrial pyruvate carrier carriers pyruvate across mitochondrial membrane into
mitochondrial matrix
• Krebs cycle (TCA)
o CoA removed from pyruvate citrate which enters Krebs cycle
• All reducing agents made in glycolysis and Krebs oxidised in mitochondrial respiratory chain during
oxidative phosphorylation (OxPhos)
OxPhos and electron transport chain (ETC)
• NADH donates electron to Q (via complex I)
– energy produced used to pump H+ out of
matrix into intermembrane space
• FADH2 not as strong electron donor (as
NADH) so donates electron to complex II
which passes electron to Q
• Q is reduced to QH2 as it accepts electron
• Complex III donates electron to cytochrome
C (CytC)
• CytC donates electron to complex IV
• H+ is pumped into intermembrane space by
CI, CIII and CIV – creates protonmotive
force (PMF) by generating a proton
gradient
o pH of matrix = very alkaline
o pH of IM space – slighlty acidic
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,BHCS2020 Summary Notes
• Complex V is ATPsynthase
• CIV passes electron to O2 (which is final electron acceptor)
o 2 H+ + ½ O2 H2O
• CV binds to H+ in IM space and pumps H+ back into matrix
o ADP + Pi ATP while H+ is pumped
• Incomplete coupling of OxPhos
o Flexible H+/O and H+/P ratios
Proton slip – decrease in pumping efficient due to partial or variable decoupling of
reaction; decreases ATP production
o Proton leak
Uncoupled respiration
H+ escapes IM space without pairing with O2
No ATP production (as H+ not pumped in through ATPsynthase)
o Electron leak
Main production of ROS
If electron reacts with O2 before CIV, O2- formed
Electron leaves complex and reacts with O2 in IM space instead of going through
respiratory chain
Mainly CI and CIII
Partial reduction of O2
• O2 + e- O2- ; superoxide is primary ROS formed by ETC
• O2- + H+ HO2 ; hydroperoxyl radical is most reactive ROS (will react with anything it contacts)
• O2 + 2H+ + 2e- H2O2 ; hydrogen peroxide is not a radical, but is very reactive – can form more
harmful species in reactions
• 2 O2- + 2H3O+ O2 + H2O2 + 2H2O
Fenton’s chemistry – role of ferric iron
• Transition metal ions can donate or accept free electrons via intracellular reactions that help create
free radicals
• Fe3+ (ferric) + O2- Fe2+ (ferrous) + O2
• Fe2+ + H2O2 Fe3+ + OH- + OH
• O2- + H2O2 OH- + OH + O2
• Fenton’s reaction – capable of generating both hydroxyl radicals and higher oxidative states of iron
causing free radical damage
Kinetics of superoxide or H2O2 production
• Superoxide production is dependent on
o [O2] availability
o 2nd rate constant (KE)
o E – enzyme with suitable electron carriers/prosthetic groups able to react with O 2
o PR – proportion of electron carrier in right form (redox state) to react with O 2
o (d(superoxide)/dt)E = KE * [O2] * PR * [E]
• Kinetics constraints
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, BHCS2020 Summary Notes
o [E] depends on
Organism Physiological state
Tissue Pathological state
Age
o Redox form of electron donor (PR) – most significant
Time-averaged parameter – subject to temporal changes
Affected by anything that alters reduction potential (Eh) of electron carrier or
prosthetic group
Depends on
• Enzyme inhibition • Post-translation
• Enzyme damage modifications
• DNA mutation • PMF
o Oxygen availability
d(superoxide)/dt = [O2] IC [O2] gradients
EC [O2] varies O2 solubility varies
o Rate constant
Superoxide, hydroxyl radical and singlet oxygen have expectational oxidising power
Singlet oxygen – in high energy state as electron in different spin (to unpaired electron in superoxide) so
more reactive than superoxide (but less than hydroxyl radical)
Nitric oxide (NO)
• Formed by NO synthases (NOS)
• By product of L-arginine L-citrulline
• NO is a neurotransmitter
• eNOS – primary signal generator in control of vascular tone, insulin secretion and airway tone,
involved in regulation of cardiac function and angiogenesis
• nNOS – development of NS, retrograde NT, important in memory and learning, regulation of
cardiac function and peristalsis
• iNOS – produces large amounts of NO as defence mechanism
• O2- + NO ONOO- ; peroxynitrite
o ONOO- has series of reactions which make more RNS
Oxidative stress
• Nucleic acid damage = DNA mutations
o Oxidative mutation of guanine – biomarker of oxidative stress
o Oxidised guanine misread at thymine so paired with adenine, which is then paired with T
• Lipid peroxidation
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