lecture 5 Flashcards
catabolic processes involve
glucose fatty acids, & amino acids
tricarboxylic citric acid cycle
TCA, Krebs
citric acid cycle is
amphibolic with roles in both catabolism and anabolism
citric acid cycle is a central pathway for
recovering energy from several metabolic fuels
citric acid cycle intermediates can also serve as
precursors for biosynthetic pathways
anaplerotic reactions
reactions that replenish intermediates depleted by other reactions
anabolic use of TCA cycle intermediates
gluconeogenesis
lipid biosynthesis
amino acid biosynthesis
porphyrin biosynthesis
gluconeogenesis
malate -> OAA->-> glucose
lipid biosynthesis
citrate-> OAA + Acetyl CoA->-> lipids
Amino Acid biosynthesis
OAA and alpha-ketoglutarate
poryphyrin biosynthesis
succinyl CoA
TCA cycle takes place in the
mitochondria
outer membrane
permeable to anything smaller than 5 kD
inner membrane
permeable to only O2, H2O and CO2
what do the membranes do
transport proteins required for everything else
pyruvate from glycolysis is split
acetyl group added to coenzyme A
-generates one NADH
-generates one CO2
Acetyl-CoA combines with oxaloacetate to form citrate
citrate enters the “cycle”
one 2-Carbon acetyl group of citrate is oxidized
-eight reactions
-produces 2 CO2
-produces 3 NADH, 1 FADH2
-1 GTP/ATP
-regenerates oxaloacetate
preparation : generation of Acetyl-CoA
Coenzyme A(CoA-SH)
derived from B-vitamins
Coenzyme A recieves 2 carbons from pyruvate in form of acetyl group
“high energy” thioester linkage
multi-enzyme process
catalyzed in 5 sequential reactions
oxidative decarboxylation
pyruvate dehydrogenase complex (E 1,2,3)
large negative delta G
produces CO2
pyurvate dehydrogenase multienzyme complex is totally awesome
multiple copies of 3 enzymes
pyruvate dehydrogenase
dihydrolipoyl transacetylase
dihydrolipoyl dehydrogenase
Ca. 25 nm
E1 pyruvate dehydrogenase
uses thiamine pyrophosphate (TPP) as co-factor
attacks carbonyl C2 of pyruvate, releases CO2
TPP remains bound to hydroxyethyl group
lipolamide arm of dihydrolipoyl transacetylase
Lipoic acid: cofactor covalently bound to Lys of E2
creates long, flexible rm can move between active sites of all 3 PDH enzymes
disulfide bond can be reduced to SH + SH
can serve as electron carrier and acetyl carrier
lipoic acid
cofactor covalently bound to Lys to E2
E2: Dihydrolipoyl transacetylase
lipo side chain extends to E1
transfers hydroxyethyl from TPP to dihydrolipoamide
partial reduction creates acetyl group
second reduction transfers acetyl group to CoA
generation of Acetyl CoA
CoA recieves 2 carbons from pyruvate in form acetyl group
“high energy” thioester linkage
E3 dihydrolipoyl dehydrogenase resets the system
catalyzes the regeneration of disulfide(oxidized) form of Lipomide/lipolysine
-Uses bound cofacter FAD (reduced to FADH2)
NAD+ oxidizes FADH2 to regenerate FAD
NAD becomes reduced (NADH +H+)
PDH enzyme complex is regenerated
NADH and H+ are made
PDH “substrate channeling”
intermediates never leave the complex tethered by lipoamide arm
acetyl CoA is also derived from fatty acids
thiol group of CoA-SH caries out nucleophilic attack
step one: creating a fatty acyl-CoA
step 2 beta-oxidation
each 4-step “pass” removes one acetyl group (2 C) from chain to make Acetyl CoA
also makes FADH2 and NADH/H+
formation of citrate from acetyl CoA and oxaloacetate
methyl C of acetyl group group is joined to carbonyl C of OAA
free CoA- SH (reduced) goes back to PDH complex
2 formation of isocitrate from citrate A 2-step process
positive delta G
transfers C2-OH of citrate to C3
formation of cis-aconitate C2-C3 double bond intermediate (tricarboxylic acid)
- Oxidation of isocitrate
a 3- step oxidative decarboxylation
initial oxidation produces oxalosuccinate intermediate
-produces first NADH (or NADPH)
decarboxylation gives alpha-ketogluterate
-enol intermediate gets rearranged
-produces first CO2
manganese ion in enzyme active site helps stabilize intermediates
4 Oxidation of alpha-ketogluterate
alpha-ketogluterate dehydrogenase complex
very similar to pyruvate dehydrogenase
-multi-enzyme complex , TPP, FAD, NAD cofactors
-produces succinyl-CoA instead of acetyl-CoA
-produces second NADH, second CO2
- Conversion of Succinyl-CoA to succinate
succinylcholine- Co synthetase
produces GTP or ATP ( depends on S-CoA-synth isozyme) and CoSH
energy released in breakage of thioester bond drives GTP/ATP formation
net delta G is -2.9 kJ/mol
- Oxidation of succinate
succinate dehydrogenase
FAD covalently bound to enzyme, along with Fe-S centers
-electrons flow from FAD-Fe/S - ETC
-ultimately leads to ATP production (oxidative phosphorylation)
succinate dehydrogenase is embedded in inner mitochondrial membrane
- hydration of fumarate
H2O added across fumarate double bond
only works with transfumarate (not cis)
- oxidation of malate
malate dehydrogenase
hydride ion transfer from malate to NAD+ makes 3rd NADH
regenerates OAA for another cycle
products of the citric acid cycle
3 NADH
1FADH2
1 GTP/ATP
2CO2
citric acid cycle regulation: regulated at the three exergonic steps
rate limiting steps(-delta G)
citrate synthase:
-inhibited by citrate NADH and succinyl-CoA
isocitrate dehydrogenase
inhibited by ATP, NADH
activated by CA2+ and ADP
alpha-ketogluterate dehydrogenase
inhibited by NADH and succinyl-CoA
Activated by Ca2+
citric acid cycle regulation
ATP, NADH acetyl-CoA = inhibitory
ADP, NAD+, CoA, Ca2+ = stimulatory
regulated at the three exergonic steps
