WEEK 2: Citric acid cycle Flashcards
what cells does the CAC take place in?
eukaryotic cells
where in the cells does the CAC take place?
mitochondrial matrix
what is the central role of the CAC?
links other biochemical pathways to the mitochondrial electron transport system (ETS)
why is the CAC an amphibolic pathway?
has both catabolic functions and anabolic functions
what catabolic functions does the CAC have?
break down of metabolites to generate ATP, NADH and FADH2
what anabolic functions does the CAC have?
supplies precursors for other pathways
how is the CAC ‘rewired’ in macrophages and dendritic cells when they respond to proinflammatory stimuli?
- glycolysis increases
- oxidative phosphorylation decreases
- CAC intermediates accumulate and are diverted to support the immune cell function
Acetyl-CoA is involved in the first reaction of the CAC. Where does the substrate come from?
From the Pyruvate Dehydrogenase (PDH) reaction
From Fatty Acid beta-oxidation
From Amino Acids reactions
From ketone bodies formed in the liver when insulin levels are low
what is coenzyme A?
an acyl carrier
how is coenzyme A an acyl carrier?
it has a thiol group as a reactive part of the molecule - this binds the acyl groups as thiol esters
what occurs in the 1st reaction of the CAC?
oxaloacetate condenses with acetyl CoA to form citrate
what enzyme catalyses reaction 1?
citrate synthase
what occurs in the 2nd reaction of the CAC?
2a and 2b. Citrate is isomerized to isocitrate
what is the intermediate compound of the second reaction?
cis-aconitate
what enzyme catalyses reaction 2?
aconitase
what occurs in the 3rd reaction of the CAC?
Isocitrate is oxidised and decarboxylated to alpha-ketoglutarate
what enzyme catalyses reaction 3?
isocitrate dehydrogenase
what is the intermediate compound of the third reaction?
oxalosuccinate
what enzyme catalyses reaction 4?
alpha-ketoglutarate dehydrogenase complex
what occurs in the 4th reaction of the CAC?
Succinyl-CoA is formed by the oxidative decarboxylation of alpha-ketoglutarate
Which of these represents the overall equation of the citric acid cycle in animal cells?
1. Acetyl-CoA+ 2H2O + 3NAD+ + FAD + GDP + Pi -> 2CO2 + 3NADH + FADH2 + CoA-SH + GTP
- Acetyl-CoA -> 2CO2 + 3NADH + FADH2 + CoA-SH + GTP
- Acetyl-CoA+ H2O + NAD+ + FAD + GDP + Pi -> CO2 + NADH + FADH2 + CoA-SH + GTP
- Acetyl-CoA+ 2H2O + 3NAD+ + FAD + ADP + Pi -> 2CO2 + 3NADH + FADH2 + CoA-SH + ATP
1.
Which process generates ATP from ADP and inorganic phosphate (Pi) in glycolysis and the citric acid cycle (via GTP)?
1. Oxidative phosphorylation
- Photophosphorylation
- Cyclic phosphorylation
- Substrate-level phosphorylation
4.
Which compounds have stored the majority of the chemical energy from glucose, by the end of glycolysis, pyruvate dehydrogenase reaction and the citric acid cycle?
1. ATP and CO2
- NADH and FADH2
- NAD+ and FAD
- ATP
2.
what is citrate?
a tricarboxylic acid
what occurs in the 5th reaction of the CAC?
Generation of a high energy phosphate from the hydrolysis of succinyl CoA
what enzyme catalyses reaction 5?
succinyl CoA synthetase with an intermediate formation of succinyl phosphate
how can GTP form ATP?
GTP may be used to form ATP in a reaction catalysed by nucleoside diphosphokinase:
GTP + ADP ATP + GDP
what occurs in the 6th step of the CAC?
Oxidation of succinate to fumarate
what enzyme catalyses the 6th step?
succinate dehydrogenase
what is the electron carrier in step 6? What is it’s reduced form?
FAD.
FADH2
what occurs in the 7th step?
addition of water to fumarate to form malate
what enzyme catalyses step 7?
fumarase
how does the double bond turn into a simple covalent bond from fumarate to malate in step 7?
a water molecule is added
what occurs in step 8 of the CAC?
oxidation of malate to regenerate oxaloacetate
what enzyme catalyses step 8?
malate dehydrogenase
how much energy is available in one mole of acetyl-CoA?
228kcal/mol
how much energy is contained in the products of CAC cycle?
207kcal/mol
what is the % of energy conservation of CAC
90 conservation
what is the energy change for the TCA cycle
~ -13kcal/mol = energy lost as heat
what best describes the purpose of the CAC?
oxidises the C atoms from acetyl groups to CO2, stores some of the energy from acetyl groups into reduced electron carriers (v important) and produces some ATP
make fc of cac as a source of biosynthetic precursors
what are anaplerotic reactions?
replenishing pathways/reactions
cac inhibitors
how many reactions are dehydrogenation reactions?
4 out of 8
what reduced cofactors could be produced from a dehydrogenation reaction?
NADH or FADH2
how is the formation of ATP different in animal cells and plant cells in step 5?
animal cells produce GTP from the first reaction which then reacts with ADP to produce ATP.
Plant cells form ATP directly
what are the 2 phases of CAC?
introduction and oxidation of two carbons to carbon dioxide
regeneration of oxaloacetate
overall eq for CAC:
Overall equation (x2 for 1 glucose)
Acetyl-CoA+ 2H2O + 3NAD+ + FAD + GDP + Pi ->
-> 2CO2 + 3NADH + FADH2 + CoA-SH + GTP
how is most the ATP produced here?
oxidative phosphorylation
what is NAD+? What does it receive?
co-enzyme of dehydrogenases. ELECTRON CARRIER involved in the oxidation of many metabolites.
Receives 2 electrons and 1 proton
what is FAD? What does it receive?
Prosthetic group of flavoproteins. TRANSFERS electrons as hydrogen atoms.
Receives 2 electrons and 2 protons
what is the reduced form of NAD+?
NADH + H+
what is the reduced form of FAD?
FADH2
the oxidation of one NADH in the elcetron chain produces how much ATP?
2.5 moles ATP
the oxidation of one FADH2 in the elcetron chain produces how much ATP?
1.5 moles ATP
what is the total yield of ATP from the oxidation of one acetyl CoA molecule?
10 ATP
3 NADH –> 3 x 2.5 = 7.5ATP
1 FADH2 –> 1 x 1.5 = 1.5ATP
1 GTP –> = 1ATP
—————————-
Total =10 ATP
which process generates ATP from ADP in glycolysis and in the citric acid cycle?
substrate-level phosphorylation
which compounds have stored the majority of the chemical energy from glucose by the end of glycolysis, pyruvate dehydrogenase and CAC?
NADH and FADH2
why is the CAC regulated?
to maintain ATP homeostasis. This specific pathway makes the link between all catabolic pathways in the cell and ETS.
how is the rate of the CAC regulated?
to correspond to the rate of the ETS
how is the rate of the ETS regulated?
by the ATP/ADP ratio and rate of ATP usage
which points are the main control points of the pathway?
production of citrate
all other 3 reactions that produce NADH
what inhibits citrate synthase?
citrate
what inhibits isocitrate dehydrogenase?
NADH
what activates isocitrate dehydrogenase?
high concs of ADP
what activates alpha-ketoglutarate dehydrogenase?
Ca2+
what inhibits alpha-ketoglutarate dehydrogenase?
NADH
what inhibits malate dehydrogenase?
NADH (inhibits all dehydrogenases)
why is calcium an an activator important for muscle cells?
- their function energy is required in the form of ATP.
- Ca2+ increases the rate of the CAC, and the rate of production of reduced electron carriers
- More reduced electron carriers provided for oxidative phosphorylation
- coordinated with the activation of muscle contraction
HOW is citrate synthase inhibited?
increased conc of citrate implies low oxaloacetate. Enzyme switched off - then acetyl-CoA is diverted to other pathways, eg for fatty acid synthesis
HOW does NADH inhibit malate dehydrogenase?
the malate enzyme is influenced by NADH/NAD+ ratio.
- low levels of NADH switches enzyme on
how is O2 linked to CAC?
- O2 is needed to regenerate NAD+
- lack of O2 leads to build up of NADH
- increased lactate in the heart muscle - angina (the heart muscle does not normally work anaerobically
why does the conc of lactate increase in muscle cells during hypoxia?
due to reoxidation of NADH from glycolysis
what is the purpose of anaplerotic reactions?
to replenish the CAC intermediates
what happens to the carbon atoms from sugars, fatty acids and amino acids during the process of cell respiration?
they get fully oxidised and released as CO2
