EXAM 3: CAC Flashcards
cellular respiration
complete oxidation
cells consume O2 and produce CO2
captures energy stored in lipids and amino acids
3 stages of cellular respiration
acetyl-coa production
acetyl-coa oxidation
electron transfer and oxidative phosphorylation
glycolysis location
cytosol
citric acid cycle location
mitochondrial matrix (except succinate dehydrogenase on inner membrane)
oxidative phosphorylation location
inner membrane
pyruvate to acetyl-CoA
required for pyruvate from glycolysis to enter CAC
not required for fatty acid catabolism
result of reaction
pyruvate —> acetyl-Coa
oxidative decarboxylation of pyruvate
first carbons of glucose to be fully oxidized
what catalyzes pyruvate to acetyl-coa
pyruvate dehydrogenase complex
pyruvate dehydrogenase complex
5 cofactors
prosthetic groups: TPP, lipoyllysine, FAD
coenzymes: NAD+, CoA-SH
coenzyme A
not a permanent part of enzymes structure
associate, fulfill reaction, dissociate
in this rxn, carries and accepts acetyl groups
pyruvate dehydrogenase complex (PDC)
multiple copies of 3 enzymes
advantages of multienzyme complex in PDC
short distance between catalytic sites, allows channeling of substrates from one catalytic site to another
channeling minimizes side reactions
regulation of activity of one subunit affects entire complex
sequence of events in oxidative decarboxylation of pyruvate
step 1: pyruvate is decarboxylated to an aldehyde; prosthetic group TPP attaches
=hydorxyethyl TPP
step 2: lipoyl cofactor is reduced (disulfide bonds break) and binds the aldehyde to form thioester
oxidizes aldehyde
step 3: first main product of acetyl-CoA
step 4: reduced lipoyllysine is reoxidized (recycled) with reduction of FAD to FADH2
step 5: regeneration of oxidized FAD cofactor forms reduced NADH (product 2)
net result of citric acid cycle
acetyl-CoA + 3NAD+ FAD + GDP + Pi + 2H2O
2 CO2 + 3NADH + FADH2+ GTP + CoA + 3H+
energy captured by electron transfer to NADH and FADH2
GTP can be converted to ATP
citric acid cycle name
citrate is made first
TCA cycle name
tricarboxylic acid cycle; first 2 molecules made have 3 carboxyl groups
citrate, isocitrate
krebs cycle name
hans krebs in 1937
CAC: Step 1
acetyl-CoA + oxaloacetate + H2O —> citrate + CoA-SH
C-C bond formation by condensation of acetyl-CoA and oxaloacetate
citrate synthase
CAC Step 1: citrate synthase reaction
condensation of acetyl-Coa and oxaloacetate
rate limiting step of CAC
activity largely depends on [oxaloacetate]
favorable, irreversible (regulated by substrate availability and product inhibition)
ordered sequential; oxaloacetate then acetyl-coa
CAC: Step 2
citrate —> cis-aconitate + H2O —> isocitrate
isomerization by dehydration / rehydration
aconitase
CAC Step 2: aconitase reaction
isomerizes molecule by removing then adding h2O
- elimination of H2O from citrate gives cis c=c bond
- addition of H2O to cis-aconitrate is stereospecific
- citrate = tert alc; poor substrate for oxidation
- isocitrate = secondary alc,good substrate
unfavorable,reversible; low product
why is isocitrate a better substrate for oxidation?
2 hydrogens to remove; one on the alc, and one on the alcohol the carbon is attached to
CAC: Step 3
isocitrate + NAD+ —> a-ketoglutarate + NADH + CO2
oxidation decarboxylation of isocitrate
isocitrate dehydrogenase
CAC Step 3: isocitrate dehydrogenase reaction
catalyzes oxidative decarboxylation
- generates NADH
- lose carbon as CO2 (complete oxidation)
oxidation converts alcohol to ketone; transfers hydride to NAD+
carboxyl group on C3 leaves
favorable, irreversible; product inhibition and ATP allosteric regulation
CAC: Step 4
a-ketoglutarate + CoA-SH + NAD+ —> succinyl-CoA + CO2 + NADH
final oxidative decarboxylation
alpha-ketoglutarate dehydrogenase complex
CAC Step 4: alpha-ketoglutarate dehydrogenase rxn
net full oxidation of all carbons of acetyl-CoA
after two turns: complete oxidation of all carbons from glucose
ketone oxidized to thioester; NAD+ reduced
succinyl-CoA has a higher energy thioester bond
favorable, irreversible; product inhibition
a-ketoglutarate dehydrogenase
complex similar to pyruvate dehydrogenase
same coenzymes, identical mechanisms; active sites different for diff sized substrates
CAC: Step 5
succinyl-CoA + GDP + Pi —> GTP + CoA-SH + succinate
generation of GTP through thioester bond cleavage
succinyl-CoA synthetase
CAC Step 5: succinyl-CoA synthetase reaction
substrate level phosphorylation
energy of breaking thioester bond allows incorporation of Pi to make GTP
- Pi displaces CoA to make succinyl phosphate
- Pi transferred to make phospho-enzyme intermediate; succinate leaves
- Pi added to GDP to produce GTP
can be converted to ATP
favorable, reversible; low product
Step 5 isozymes in brain, muscle, heart
use ADP, make ATP
Step 5 isozymes in liver, kidney
GTP, GDP
CAC: Step 6
succinate + FAD —> FADH2 + fumarate
oxidation of alkane to alkene
succinate dehydrogenase
CAC Step 6: succinate dehydrogenase rxn
in inner membrane
oxidation of alkane to alkene; reduction of FAD to FADH2
reversible; low product concentration
why isn’t NAD+ used in step 6 in the succinate dehydrogenase reaction?
reduction potential is too low
CAC: Step 7
fumarate + OH- —> carbanion TS + H+ —> L-malate
hydration across a double bond
fumarase
CAC Step 7: fumarase reaction
stereospecific
addition of water is trans, forms L-malate
- OH- adds fumarate first, then H+ to carbanion
- cannot distinguish between inner carbons, so either can gain OH-
favorable, reversible; low product
CAC: Step 8
L-malate + NAD+ —> NADH + H+ + oxaloacetate
oxidation of alcohol to ketone
malate dehydrogenase
CAC Step 8: malate dehydrogenase reaction
final step
alcohol on malate oxidized to ketone; NAD+ reduced
regenerates oxaloacetate for citrate synthesis
unfavorable, reversible; oxaloacetate kept very low by citrate synthesis to pull reaction forward
how many NADH are made for CAC?
3 per cycle; 6 per glucose
how many FADH2 made per CAC?
1 per cycle; 2 per glucose
how many GTP/ATP made per CAC***?
1 per cycle; 2 per glucose
how many Co2 formed in CAC?
2 per cycle; 4 per glucose
4 reactions in CAC with regulation
pyruvate dehydrogenase
citrate synthase
isocitrate dehydrogenase
alpha-ketoglutarate dehydrogenase
regulatory mechanism in CAC
activation: substrate availability
NAD+, AMP, ADP
inhibition: product accumulation
ATP, NADH
enzymes may be associated to allow channeling
amphibolic intermediates
intermediates can go into other pathways
but if they are taken out, cycle isn’t completed, and oxaloacetate will run out
catabolic
further break down
anabolic
makes other molecules
anapleurotic reactions
intermediates in CAC can be used in biosynthetic pathways (removed from CAC)
must replenish the intermediates in order for the cycle and central metabolic pathway to continue
3 carbon intermediates (pyruvate, phosphoenolpyruvate) are carboxylated to form 4 carbon intermediates (oxaloacetate, malate)
malate is one step away from oxaloacetate
