LECTURE 10 - CITRIC ACID CYCLE Flashcards
what is the definition of the CAC?
series of 8 enzymatic reactions that combine acetyl CoA with oxaloacetate to generate CO2, NADH and FADH2 and regenerates the starting product oxaloacetate
is the CAC aerobic or anaerobic?
neither, does not directly consume oxygen, but part of aerobic metabolism
what does amphibolic mean?
site of anabolism and catabolism
what are cataplerotic reactions
depleting the CAC intermediates
what are anaplerotic reactions
replenishing the depleted CAC intermediates
what is the overall reaction of the CAC?
3NAD+FAD+GDP+PI+AcetylCoA—————>3NADH+FADH2+GTP+CoA+2CO2
which steps confer directionality to the citric acid cycle?
reaction 1: citrate synthase
reaction 3: isocitrate dehydrogenase
reaction 4: alpha ketoglutarate dehydrogenase
these three enzymes catalyse irreversible reactions
what reaction does citrate synthase catalyse?
oxaloacetate+acetylCoA –> citrate
what are some key points of the citrate synthase reactions?
-there are very low levels of oxaloacetate
- the high energy bond of acetylCoA drives the reaction forward + the high energy intermediate of this reaction
- the only reaction where a C-C bond is formed
what is the reaction of aconitase?
citrate <-> isocitrate
what is the reaction of isocitrate dehydrogenase?
isocitrate + NAD+ -> alpha ketoglutarate (5C) + NADH+ H+
the first step where we make NADH and CO2
the CO2 comes from oxaloacetate, not from acetyl CoA
what is the overall reaction of ketoglutarate dehydrogenase?
alpha ketoglutarate + CoA+ NAD+ -> succinyl-CoA+CO2+NADH+H+
what are some key characteristics of the ketoglutarate dehydrogenase reaction?
- oxidative decarboxylation reaction that generates NADH and CO2
- the decarboxylation provides energy to generate succinyl CoA
- succinyl CoA is a high energy intermediate because of its thioester bond
- like the PDC, KG dehydrogenase is a multienzyme complex, E1, E2 and E3
- the E3 enzyme is the same for the PDC as for the KG dehydrogenase
what is the succinyl-CoA synthase reaction?
succinylCoA+GDP+Pi<-> succinate + GTP
use of the high energy succinylCoA to make a GTP
what was generated at the point of the succinyl CoA synthase reaction?
2NADH
2CO2
1ATP
what is the reaction of succinate dehydrogenase and some key points?
succinate + E-FAD <-> fumarate + E-FADH2
- here E=enzyme
- FAD is always bound to the enzyme by a histone residue
- FAD cannot diffuse as a free metabolite
- E-FADH2 restores FAD immediately by funneling the electrons into the ETC
- succinate dehydrogenase is actually complex II of the ETC, the only complex that does not pump protons
what is the door to get electrons to complex III?
have to get through Q first
what does fumarase do?
fumarate+H2O<-> malate
what is the malate dehydrogenase reaction?
malate+NAD+<-> oxaloacetate + NADH+ H+
what are some key characteristics of the malate dehydrogenase reaction?
- the reaction is endergonic, but in vivo it is at equilibrium because malate levels are soo much greater than oxaloacetate, which pushes towards the production of oxaloacetate
- the coupling of this reaction + the formation of citrate from oxaloacetate helps the reaction move forward
- citrate synthase rxn is highly exergonic which continues the cycle
how many ATPs are produced from one NADH?
2.5ATP
how many ATPs are produced by one FADH2?
1.5ATP
how many ATPs are produced from the PDC?
1 NADH, which means 2.5ATP
per pyruvate
5 ATP per glucose
how many ATPs are produced from the CAC?
3 NADH=7.5ATP
1FADH2=1.5ATP
1GTP=1ATP
total 10 per pyruvate
20 per glucose
what is the grand total of ATPs produced per glucose?
7 from glycolysis
5 from PC
20 from CAC
total 32 ATP/glucose
map of the CAC regulation
which enzymes are under the biggest control?
the ones with the negative delta G
what are the mechanisms present in CAC regulation?
- substrate availability
- product inhibition
- competitive feedback inhibition
- allosteric activation/inhibition
what is the difference between product inhibition and allosteric activation/inhibition?
product inhibition binds the active site
allosteric binds away from the active site, is not a product
what is the central regulator of the CAC
NADH levels regulate the whole thing
how is citrate synthase regulated?
- substrate availability:
the reaction rate varies with the concentration of the two precursors: acetyl CoA and oxaloacetate
and acetyl CoA is regulated by the PDC - product inhibition:
citrate is a competitive inhibitor of oxaloacetate for binding to citrate synthase - competitive feedback:
succinyl CoA competes with Acetyl CoA (they both have the CoA component) - allosteric inhibition:
NADH - allosteric activation”
ADP
how is isocitrate dehydrogenase regulated?
- product inhibition:
NADH displaces NAD+ - allosteric activation:
ADP and Ca2= - when isocitrate dehydrogenase is off:
isocitrate and citrate go up
citrate goes to the cytoplasm
which activates acetyl CoA carboxylase
activates fatty acid synthesis
inhibits PFK, inhibits glycolysis
a surplus of citrate is an indicator of high energy charge
how is alpha ketoglutarate dehydrogenase regulated?
- product inhibition:
inhibited by NADH, succinyl CoA - allosteric activation by Ca2+
what are the 3 enzyme outside of the CAC regulated by CAC intermediates?
- PFK inhibited by citrate
- pyruvate dehydrogenase is inhibited by acetyl CoA and NADH
- pyruvate carboxylase is activated by acetyl CoA
how does the energy charge of the cell regulate the CAC?
NADH plays a central role because reoxidation is coupled to oxygen consumption and ATP synthesis
in need for ATP, greatest flux of CAC
in abundance of ATP and NADH, intermediates will be used for other purposes
where does citrate go when the energy charge is high?
goes to the cytosol
stimulates synthesis of fatty acids (activates acetyl CoA carboxylase)
downregulates glycolysis (inhibits PFK)
how does the carbon count work out in the CAC?
first reaction: Acetyl CoA (2C) and oxaloacetate (4C) make a 6 carbon citrate
over the course of the cycle, 2 carbon dioxides are lost, which means that you’re back to the 4C oxaloacetate
