11 - citric acid cycle and electron transport chain Flashcards
what are the three stages of glucose oxidation?
- Glycolysis
- TCA cycle
- Electron transport chain
what is important about the TCA cycle?
the cycle can only work if there is sufficient oxidising capability in the cell (e.g. final electron acceptor = oxygen).
no direct oxygen reactions in the cycle
2 ATP regeneration directly (substrate level)
main job to produce: reduce cofactors - 6 NADH+ H+ and 2 FADH2
these, plus 2 NADH+ H+ from pyruvate can pass their 28 electrons and protons to the electron transport chain, via 2 electron hydride transfer, to generate 28 ATP (+2 ATP from substrate level P= 30 ATP)
explain the citric acid cycle
also known as tricarboxylic acid cycle (TAC) or Kerbs cycle.
aerobic energy metabolism
a cycle of 8 reactions in which a 6 carbon molecule is sequentially oxidised.
provides electrons and protons for ETC.
where does the TAC cycle take place?
in the mitochondrial matrix
what happens if oxidising capacity is available in the cell?
pyruvate molecules *enter the inner matrix of the mitochondrion
pyruvate irreversibly converts to acetyl coA.
produced reduced cofactor** and CO2.
- pyruvate translocase
** pyruvate dehydrogenase
how many enzymes are used in the TCA cycle?
8
what is the first stage of the eight steps of the TCA cycle?
1 - citrate synthesis
only step in TCA cycle that involves the formation of a C-C bond
2 carbon Acetly CoA feeds into the cycle by joining with:
- the 4 carbon molecule oxaloacetate
in a hydration reaction to create 6 carbon citrate.
what is the second stage of the eight steps of the TCA cycle?
2 - aconitase
the -OH group is moved from the 3rd to 4th carbon (dehydration and hydration)
1) generation of NADH+ H+
2) formation of CO2
what is the third stage of the eight steps of the TCA cycle?
3 - isocitrate dehydrogenase
what is the fourth stage of the eight steps of the TCA cycle?
4 - a-ketohlutarate dehydrogenase
what is the fifth stage of the eight steps of the TCA cycle?
5 - succinyl CoA synthetase
what is the sixth stage of the eight steps of the TCA cycle?
6 - succinate dehydrogenase, coupled with ETC
what is the seventh stage of the eight steps of the TCA cycle?
7 - fumarate hydratase
what is the eighth stage of the eight steps of the TCA cycle?
8 - malate dehydrogenese
what is energy provided from the oxidation fro in the ETC?
energy is provided by oxidation of the 12 molecules of NADH and FADH2.
where do the 10 NADH+ H+ that enter the ELC cone from?
- 2 from glycolysis
- 2 from the transformation of pyruvate into acetyl-CoA
- 6 from the citric acid cycle
where do the 2 FADH2 that enter the ETC come from?
the citric acid cycle
how are 32 ATP produced in the ETC?
this phase uses the electrons and protons carried by the reduced cofactors to generate ATP.
electron transport chain PLUS an ATP synthase
in eukaryotes what is located in the mitochondrial inner membrane?
the ETC and ATP sythase.
what are the 5 ETC membrane complexes?
- Complex I/NADH dehydrogenases
- Complex II/succinate dehydrogenase, couples to step 6 in TCA, FADH2 oxidation
- Coenzyme Q - Complex III/coenzyme Q-cytochrome c reductase.
- Cytochrome c - Complex IV/cytochrome c oxidase
- Complex V/ATP synthase
explain the ETC
electrons pass from NADH via complex I, and from FADH2 via complex II
the energy release from electron flow through the remaining complexes pumps protons across inner membrane from matrix to intermembrane space.
at complex III, no additional electrons enter the chain, but electrons from complexes I and II flow through
when electrons arrive at complex IV, they are transferred with protons to a molecule of oxygen
this has a proton pump activity.
since oxygen gains electrons, it is reduced to water, hydrogen is oxidised.
what is oxidative stress?
unpaired electron intermediates between O2 and H2O are all toxic free radicals: reactive oxygen species (ROS)
complexes I and II occasionally allow electrons to escape from the ETC to form ROS.
what is substrate-level phosphorylation?
phosphate and energy comes from another molecule or substrate e.g. in glycolysis.
Phosphoenolpyruvate + ADP –> pyruvate + ATP
what is oxidation phosphorylation?
phosphate comes from solution (Pi) and requires a huge amount of energy input e.g. from the proton gradient:
ADP + Pi –> ATP
what is the proton gradient?
it is sometimes called the proton-motive force or electrochemical potential and is the combination of two gradients
what is ATP resynthesis via ATP synthase?
complex V is an ATP synthase.
protons flow down their chemiosmotic gradient through ATP synthase.
conformational change in enzyme allows ADP to react with Pi and create ATP.
what is anaerobic respiration?
production of ATP in the absence of oxygen = need alternative electron acceptor
- nitrate
- iron (III)
- manganese
- sulphate
e.g. microorgansims that live in
- stagnant ponds, solid sediments, e.g. Desulfococcus used sulphate as terminal electron acceptor
- volcanic caves, e.g. snottite bacteria use sulphate
- in your colon, e.g. E.coli use nitrate as a terminal electron acceptor.
what are anaerobes?
bacterial or fungal organisms that can live without oxygen:
- obligate anaerobes - organisms cannot live in the presence of oxygen, e.g. clostridium botulinum.
- Facultative anaerobes - organisms can live with or without oxygen. they can switch metabolism depending on the conditions, e.g. saccharomyces cerevisae.
