In order to enter the Citric Acid Cycle pyruvate must:
- Pass from the cytosol of the cell into the mitochondrion (via membrane transport protein)
- Be converted to Acetyl Coenzyme A
- An acyl group carrier
- Referred to as CoA
- Written in equations as CoA-SH as the thiol group is the reactive part
- Carries acyl groups as thiol esters (high energy compounds)
Acetyl CoA formation
PYRUVATE ====(NAD+==>NADH)===> ACETYL COA
Pyruvate enters the mitcohondrial matrix and is converted to Acetyl CoA
Acetyl group enters the citric acid cycle
Enzyme used: pyruvate dehydrogenase
The overall process is oxidative decarboxylation as a carbon is removed as CO2
Two carbons are transferred as an acetyl unit
Two electrons and two hydrogens are transferred to NAD
Acetyl CoA is also generated by break down of fatty acids
Citric Acid Cycle - Synthesis of Citrate
2-C transferred to AcetylCoA are now ready to enter the cycle.
Feature: uses free energy available from destruction of the Acetyl group to drive the cycle - thermodynamically
Purpose: to produce H+ and e- that are transferred to NAD+ and FAD.
NADH and FADH2 are utitlised in the Electron Transport System to produce ATP.
ACETYL COA + OXALOACETATE ====> CITRATE catalysed by citrate synthase
Acetyl CoA enters the cycle by reaction with oxaloacetate to form citrate - requiring an input of water.
This reaction is accompanied by a loss of free energy (as heat) and is irreversible. It is a highly regulated step, as a metabolic branch point.
After one complete turn of the cycle, oxaloacetate is re-formed and the acetyl group has disappeared
Citrate to Isocitrate
CITRATE =======> ISOCITRATE catalysed by aconitase
Strategy: to switch the hydroxyl group of citrate (symmetrical) from 3 position to 2 position (asymmetrical)
Citrate is important for biosynthetic reactions
DeltaG (free energy) is negative for this reaction
Decarboxylation of Isocitrate
ISOCITRATE ===(NAD+==>NADH+CO2)===> A-KETOGLUTARATE
catalysed by isocitrate dehydrogenase
Stimulated by NAD+ and ADP
Inhibited by NADH and ATP
Commits citrate to the citric acid cycle
Decarboxylation of A-Ketoglutarate
A-KETOGLUTARATE ===(NAD+==>NADH+CO2)===> SUCCINYL-COA
catalysed by a-ketoglutarate dehydrogenase complex
Alpha-ketoglutarate is decarboxylated to form succinyl-CoA (high energy thio-ester)
Metabolic branch point, activated by low energy status of cell
Formation of Succinate
catalysed by succinyl CoA synthetase
Only reaction that generates a high energy phosphate
GTP is converted to ATP or is used in protein synthesis
SUCCINATE ===(FAD==>FADH2)===> FUMARATE
catalysed by succinate dehydrogenase
Enzyme is bound to the inner surface of the mitochondrial membrane
Directly linked to the ETS
Hydration of Fumarate
FUMARATE ===(+H20)===> MALATE
catalysed by fumarase
Dehydrogenation of Malate
MALATE ===(NAD+==>NADH)===> OXALOACETATE
catalysed by malate dehydrogenase
Reaction reforms oxaloacetate
Thermodynamically, malate is the favoured product
In the cell, oxaloacetate concentration is so low because it is constant used that the reaction proceeds anyway.
Molecules produced in the Citric Acid Cycle
2 molecules of Acetyl CoA enter the Citric Acid Cycle from one molecule of glucose
2 molecules of GTP are formed (can be converted to 2ATP)
6 molecules of NADH formed for the ETC
2 molecules of FADH2 formed for the ETC
Electron Transport System
Purpose: produce ATP from ADP and Pi using energy from electron movement to oxygen
Electrons flow along a series of electron transport carriers embedded in the membrane.
Each carrier in the series accepts and releases electrons at a lower energy level than the carrier preceding it.
Energy released is used to transport H+ across the mebrane against the concentration gradient
This maintains the H+ gradient to drive ATP synthesis.
Most carriers are:
- Integral membrane proteins and contain: a non-protein organic group and a prosthetic group containing a metal ion that accepts and releases elctrons
Complexes in the ETC
Conducts electrons from NADH to ubiquinone
Carries electrons from oxidation of succinate to ubiquinone. FADH2 enters
Conducts electrons from ubiquinone to cytochrome c
Conducts electrons from cytochrom c to oxygen. Inhibited by cyanide
End of the chain
After travelling down the ETC:
- Electrons are at lower energy levels
- Electons pass to a final electron acceptor - oxygen
- This is converted to water:
O2 + 4H+ + 4e- ===> 2H2O
The energy has been used to create a large concentration gradient of H+.
ATP synthase uses this gradient as an energy source for ATP synthesis
The Chemiosmotic Theory
As H+ ions are pumped form the matrix to the intermembrane space this generates:
a pH gradient and an electrical gradient as the matrix will be negatively charged relative to the intermembrane space.
THE PROTON MOTIVE FORCE
An electrochemical gradient has an electrochemical potential energy - the Proton Motive Force
As protons flow back, ATP is produce. Approx 3H+ ions must flow back for every ATP produced.
Energy from the flow of protons back through the inner mitochondrial membrane is used by ATP-ase to phosphorylate ADP
ATP synthase is an enzyme composed of two main usits F1 and F0.
A binding change occurs when H+ passes through the unit back into the matrix.
2,4-DNP is a weak acid, so in high H+ concentrations, it will be protonated.
If H+ concentration is low, it will lose a H+ ion.
It is found in membranes due to its hydrophobicity.
Therefore ATP synthesis decreases in the presence of 2,4-DNP.
ETC and Oxidative Phosphorylation
ETC and Oxidative Phosphorylation are coupled via the energy provided from the ETC.
If they become uncoupled (e.g. with 2,4-DNP), energy is lost as heat resulting in weight loss
Extensive uncoupling leads to liver damage and DEATH
In total, 32 ATP molecules are produced from cellular respiration