BRIDGING REACTION
STEP 2
Fall 2013
BIOT 309
TRANSITION OR BRIDGING REACTION
Connects glycolysis to citric acid/Kreb’s Cycle
OVERALL REACTION
2 pyruvate + 2 NAD+ + 2 CoA-SH (coenzyme A)
2 acetyl-CoA + 2 NADH + 2 H+ + 2 CO2
CONNECTION TO OTHER BIOLOGY: Where
else is CO2 made?
TRANSITION REACTION
3 carbon
Co A
2 carbon
STEP 3 AEROBIC RESPIRATION:
Krebs Cycle
BIOT 309
Fall 2013
Tricarboxylic Acid Cycle =
Krebs Cycle =
Citric acid Cycle
THE TCA CYCLE
• Converts acetyl CoA (from pyruvate via
bridging reaction) to CO2
• Provides small amounts of energy in the form
of GTP/ATP
• Collects electrons and stores as NADH and
FADH2  Electron Transport Chain (ETC)
• Provides intermediates for other pathways
• Occurs in cytoplasm
KREB’S CYCLE
Summary Reaction:
acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O
——>
2CO2 + HSCoA + 3NADH + FADH2 + GTP + 2H+
TCA/KREB’S
CYCLE
CITRIC ACID CYCLE
CHEMICAL REACTIONS
Key Equation:
Δ G0 = -RTlnK eq
• Δ G0 = Gibb’s standard free energy change,
distance from equilibrium, (expresses driving
force of reaction)
• K eq =[products]/[reactants]; measurable
• R= gas constant
• T = absolute temperature (Kelvin)
BIOCHEMICAL REACTIONS
• Instead of Δ G0, Δ G0’ is used
• Δ G0’ = standard free energy change at pH 7.0
= biochemical standard free energy
Remember: enzymes, cofactors
• Lower activation energy
• Accelerate reaction
• Organize and control
reaction
• Recover energy in new
chemical forms and
make it available for
other uses
Gibb’s Free Energy
• if Δ G0’ is negative, reaction goes forward
spontaneously; - products have less energy
than reactants
• if Δ G0’ is ~ 0, reaction is at equilibrium
• if Δ G0’ is positive, reaction does not go forward
spontaneously
• Δ G0’ of two or more reactions is calculated by
adding reactions and the Δ G0’ of the reactions
• CAVEAT: Δ G0 values shown in next slides will not be true under all
circumstances, could be different for prokaryotes and eukaryotes
KREB’S CYCLE, step 1
2C
4C
6C
Citrate Synthase
Aldol Condensation, X
KREB’S CYCLE, step 2
Aconitase
Dehydration, Fe-S
KREB’S CYCLE, step 3
Aconitase
Hydration, 4Fe-4S
Steps 2 & 3 combined
STEPS 2 & 3 done by one enzyme
aconitase
Observe that:
• Step 2: dehydration generates (double bond)
intermediate (cis-aconitate)
• Step 3: dehydration moves position of OH
group
PRINCIPLE & EXAMPLE:
Δ G0’ of overall reaction is calculated by adding
reactions and the Δ G0’ of the reactions*:
citrate 
cis-aconitate
cis-aconitate 
citrate 
isocitrate
isocitrate
Δ G0’ = +2 kcal/mol
Δ G0’ = -0.5 kcal/mol
Δ G0’ = +1.5 kcal/mol
• Applied to 2 or more reactions, e.g., all of
EMP or TCA
KREB’S CYCLE, step 4
NADH,
H+
NAD+
6C
SPONTANEOUS
5C
Isocitrate Dehydrogenase
2 step reaction
Oxidative decarboxylation, Mg2+ or
Mn2+
KREB’S CYCLE, step 5
NADH,
H+
NAD + +
CoA-SH
5C
SPONTANEOUS
4C
α-Ketoglutarate Dehydrogenase
Complex
Oxidative Decarboxylation, TPP,
Lipoic Acid, FAD
KREB’S CYCLE, step 6
GTP converted into
ATP by nucleoside
diphosphate kinase
Succinyl CoA Synthetase
Substrate Level Phosphorylation, FAD,
TPP, Lipoic Acid
KREB’S CYCLE, step 7
Succinate Dehydrogenase
Oxidation, FAD & FeS
Why FAD?
• alkane oxidation poorly
exergonic and can’t reduce
NAD+
KREB’S CYCLE, step 8
Fumarate Hydratase
Hydration, Fe-S
KREB’S CYCLE, step 9
Δ G0’ = +7 kcal/mol
Malate Dehydrogenase
Oxidation
<
>
>
*
KREB’S CYCLE !!!
Summary Reaction:
acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O
——>
2CO2 + HSCoA + 3NADH + FADH2 + GTP + 2H+
Transition Reaction + Kreb’s Cycle
Summary Reaction:
1 pyruvate + 4 NAD+ + 1 FAD + 1 GDP + 1 Pi
——>
4 CO2 + 4 NADH + 4 H+ + 1 FADH2 + 1 GTP(1 ATP)
EMP + TR + TCA
Summary Reaction:
GLUCOSE + 2H20 + 10 NAD+ 2 FAD +
4 ADP + 4 Pi
——>
6 CO2 + 10 NADH + 10 H+ + 4 ATP + 2FADH2
GLYOXYLATE CYCLE
KREBS CYCLE ALTERNATIVE
BIOT 309
Fall 2013
GLYOXYLATE SHUNT/CYCLE
• By-passes 2 decarboxylation steps in TCA making
possible
– net formation of succinate, oxaloacetate, and other cycle
intermediates from acetyl-CoA
• Retains the two carbons lost in decarboxylation steps
with each turn of TCA
• => net synthesis of oxaloacetate, a four-carbon
molecule, because each turn of the cycle incorporates
two molecules of acetyl-CoA
– Oxaloacetate used for other purposes
GLYOXYLATE SHUNT/CYCLE
• Allows many bacteria to metabolize twocarbon substrates such as acetate
FOR EXAMPLE:
E. coli can be grown in a medium that provides
acetate as the sole carbon source. E. coli
synthesize acetyl-CoA, then uses it for energy
production (via the citric acid cycle)
GLYOXYLATE SHUNT/CYCLE
• Some enzymes in common with TCA
• BUT has two exclusive enzymes not in TCA
– isocitrate lyase: cleaves D-isocitrate to glyoxylate and
succinate
– malate synthase: forms L-malate from glyoxylate and
acetyl-CoA
GLYOXYLATE SHUNT/CYCLE
• Used when the principal or sole carbon
source is a C2 compound (acetate, ethanol).
• Fat catabolism produces acetyl CoA which
feeds into other catabolic reactions and
produces energy