Citric Acid Cycle and Oxidative Phosphorylation Regulation of Metabolism Flashcards
cellular respiration overview and 3 major stages
cells consume O2 and produce CO2
complete oxidation of glucose (–> CO2 & H2O) provides more energy (ATP) than glycolysis (glucose –> pyruvate), also captures energy stored in lipids and AA
stage 1: acetyl CoA production
stage 2: acetyl CoA oxidation
stage 3: e- transfer and oxidative phosphorylation
3 stages of cellular respiration
1: oxidation of FA, glucose, and some AA to produce acetyl CoA, generates ADH + NADH, final product acetyl CoA, e- donor is NADH.
2: oxidation of acetyl groups in the citric acid cycle, includes 4 steps in which e- are abstracted. remaining CO2 are released from CHO, AA and FFAs
3: oxidative phosphorylation, generates vast majority of ATP during catabolism.
e- carried by NADH and FADH2 are funnelled into a chain of mitochondrial E0 carriers, ultimately reducing O2 to H2O, this e- flow drives the production of ATP
chemiosmotic mechanism for ATP synthesis
in eukaryotes, stages 2 and 3 are localised to the mitochondria, glycolysis occurs in the cytoplasm
- citric acid cycle occurs in the mitochondrial matrix
- oxidative phosphorylation occurs in the inner mitochondrial membrane
conversion of pyruvate to acetylCoA
final step in stage 1 of respiration, highly thermodynamically favourable, irreversible reaction.
- function of CoA is to accept and carry acetyl groups
- net reaction: oxidative decarboxylation of pyruvate, first carbons of glucose to be fully oxidised (released as CO2)
catalysed by the pyruvate dehydrogenase complex (PDC)
- requires 5 coenzymes
pyruvate dehydrogenase complex (PDC)
PDC catalysed confession of pyruvate to acetylCoA, PDC is a large multi enzyme complex.
advantages of multi enzyme complexes: short distance between catalytic sites allows channeling of substrates from one catalytic site to another, minimises side reactions, regulation of activity of one subunit effects the entire complex
conversion of pyruvate to acetylCoA
highly favourable, irreversible reaction, generates NADH, produces CO2
role of the citric acid cycle
At each turn of the cycle, three NADH, one FADH2, one GTP (or ATP), and two CO2 are released in oxidative decarboxylation reactions (reversible reactions)
events of the citric acid cycle
Step 1: C-C bond formation between acetate (2C) and oxaloacetate (4C) via condensation to make citrate (6C)
& CoA-SH
Step 2: Isomerization via dehydration/rehydration converts citrate to isocitrate
Steps 3 & 4: Oxidative decarboxylations to give 2 NADH and CO2, regulated by [ATP]
Step 5: Substrate-level phosphorylation to give GTP (GTP converted to ATP)
Step 6: oxidation of alkane to alkene, via dehydrogenation to give FADH2
Step 7: Hydration across a double bond, product concentration kept low to pull reaction forward
Step 8: oxidation of malate and regeneration of oxaloacetate via dehydrogenation to give NADH
net result of the citric acid cycle
acetylCoA + 3NAD+ + FAD + GDP + Pi + 2H2O –> 2CO2 + 3NADH + FADH2 + GTP + CoA + 3H+
Net oxidation of two carbons to CO2, equivalent to two carbons of acetyl-CoA
Energy captured by electron transfer to NADH and FADH2, Generates 1 GTP, which can be converted to ATP
Completion of cycle
regulation of the citric acid cycle
Regulated at highly thermodynamically favorable and irreversible steps
PDC (pyruvate to Acetyl CoA),
Citrate Synthase, IDH, and KDH (steps 1,3 & 4, CAC)
General regulatory mechanism
o activated by substrate availability
o inhibited by product accumulation
Overall products of the pathway are NADH and ATP
affect all regulated enzymes in the cycle
- activators: NAD+ and AMP
- inhibitors: NADH and ATP
energy from reduced fuels is used to synthesise ATP
Carbohydrates, lipids, and amino acids are the main reduced fuels for the cell.
Electrons from reduced fuels are transferred to reduced cofactors NADH or FADH2.
In oxidative phosphorylation, energy from NADH and FADH2 is used to make ATP.
NADH & FADH2 transfer e- gained while oxidising other molecules to the Electron Transport Chain (ETC)
key electron carriers
NAD+: nicotiadamide adenine dinucleotide
- phosphorylation: NADP+
- reduction to NADH and NADPH
- accepts H+ ion from an oxidisable substrate
FADH: flavin adenine dinucleotide
NADH and FADH2 are the main products of glycolysis and the citric acid cycle
oxidative phosphorylation utilised the reduced co-enzymes to produce ATP via ETC
NADH = 2.5 ATP FADH2 = 1.5 ATP
chemiosmotic theory
ADP + Pi –> ATP
highly thermodynamically unfavourable
- phosphorylation of ADP not a result of a direct reaction btwn ADP and high energy phosphate carrier
- energy need to phosphorylate ADP is provided by the flow of protons down the electrochemical gradient
- energy released by e- transport is used to transport protons against the electrochemical gradient
energy coupling requires a membrane that is impermeable to ions, is either the plasma membrane in bacteria or inner membrane in mitochondria
- membrane must contain protons that couple the downhill flow of e- in the e- transfer chain with the uphill flow of protons across the membrane
- membrane must contain a protein that couples the downhill flow of protons to the phosphorylation of ADP
NADH and FADH2 movement in membrane
Electrons move spontaneously through a chain off membrane-bound carriers, the respiratory chain, driven by the high reduction potential of oxygen and the relatively low reduction potentials of the various reduced substrates (fuels) that undergo oxidation in the mitochondrion. Electron flow creates an electrochemical potential by the transmembrane movement of protons and positive charge. This electrochemical potential drives ATP synthesis by a membrane-bound enzyme, ATP synthase, that is fundamentally similar in mitochondria and chloroplasts, and in bacteria and archaea as well
structure of mitochondrion
double membrane leads to four distinct compartments
- outer membrane: porous, allows passage of metabolites
- inter membrane space (IMS): higher [proton], low pH
- inner membrane: impermeable, protein gradient across it, location of ETC complexes, convolutions called cristae serve to inc the SA
- matrix: location of citric acid cycle and parts of lipid and AA metabolism, lower [proton] and higher pH