BCHM-3050 Lecture Notes - Lecture 13: Carboxy-Lyases, Hydration Reaction, Isocitric Acid
26 Jun 2018
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Chapter 13: The Citric Acid Cycle
Oxidative Processes
Circular Pathway – citric acid cycle (TCA Cycle/Krebs Cycle) is the central aerobic pathway for
oxidizing all metabolic fuels. Entry to this pathway is acetyl coA and pyruvic acid.
Entry of fuel as pyruvate from glycolysis into the citric acid cycle
Most of the energy yield from the oxidation of substrates in the citric acid cycle is stored in reduced
electron carries such as NADH and FADH2.
3 Stages of Aerobic Respiration
In stage 1.5, the carbon from metabolic fuels is incorporated into acetyl coA via PDH. Acetyl coA is
what enters the citric acid cycle.
oStage 1 = glycolysis
In stage 2, the citric acid cycle oxidizes actyl-coA to product CO2, reduced electron carriers, and a
small amount of ATP.
In stage 3, the reduced electron carriers are re-oxidized via the ETC providing energy for synthesis of
additional ATP.
Eukaryotic Mitochondrial Structure
Stages 1.5 and 2 of respiration occur in the mitochondrial matrix. Reactions of the 3rd stage are
catalyzed by membrane-bound enzyme complexes in the inner mitochondrial membrane.
** Glycolysis takes place I the cytosol of eukaryotes **
Mitochondria have their own single-stranded DNA. “mtDNA” encodes many of the proteins needed for
the citric acid cycle and the ETC, but not all of them.
They have their own ribosomes for mRNA translation, and mtDNA is ONLY passed down through
MOTHERS.
They can divide and merge depending on cellular conditions.
Stage 1.5:
Pyruvate dehydrogenase multienzyme complex converts to pyruvate to acetyl-coA using a variety of
cofactors.
Pyruvate and NAD+ and CoA + SH makes actyl-coA and NADH + CO2.
oReaction is favorable
3 enzymes make up the PDH complex (PDC):
1. Pyruvate Dehydrogenase (E1)
2. Dihydrolipoamide transacetylase (E2)
3. Dihydrolipoamide Dehydrogenase (E3)
KNOW FUNCTIONS OF 5 COFACTORS (CHART)
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Thiamine Pyrophosphate (TTP)
Functions because of the thiazole ring
TPP is a prosthetic group bound to the E1 subunit of PDH
Unlike B-keto acids, a-keto acids develop an unstable carbanion intermediate during carboxylation.
oPyruvate is the simplest of the a-keto acids
oLose a carbon
TPP catalyzes the cleavage of C-C bond by stabilizing this carbanion intermediate by covalent
catalysis.
Pyruvate comes in and binds to TPP, and goes through decarboxylizes, and remaining 2 carbons (actyl
group) are passed to lipoamide and goes to E2 subunit.
Lipoamide
Is an electron carrier and acyl group carrier.
Is a coenzyme located on the E2 subunit that participates in the transfer of acyl groups.
oCan have 2 forms – oxidized and reduced
oIf disulfide linkage on the head = oxidized (lipoamide)
oHydrogens to each sulfur atom = reduced (dyhydrolipoamide)
oActyl-dihydrolipoamide = reduced form of lipoamide. Carries actyl group to awaiting CoA-SH
molecule. Has ester bond
The 14 A long carbon chain acts as a swinging arm to interact with the E1 and E3 subunits.
Coenzyme A
Is what is awaiting for the acyl group, and activates the acyl groups for entry into the TCA cycle.
Free thiol of B-mercaptoethylamine reacts to form thioesters, like acetyl-coA. The products are energy
rich compounds.
Coenzyme A contains a THIOL GROUP on the end and use coenzyme A and acetic acid to make acetyl
co-A. You make a thiol ester (acetyl coA) and use this to store energy for the next stage.
Thioesters
The larger the atomic size of the thioester sulfur atom results in poorer stabilization by resonance.
Therefore, the thioester has a higher potential energy for acyl group transfer than the ester.
***SO FAR, 3 PROTHESTIC GROUPS – TTP, LIPOAMIDE AND FAD. COENZYME A IS A
TRANSIENTLY BOUND ENZMYE AND USED TO PICK UP ACYL GROUP – NOT BOUND TO PDH
*****
~ Physically bound to either E1, E2, or E3.
Riboflavin, FMN, and FAD
FAD is the 4th coenzyme/prosthetic group
**FAD is the indicator of PDH ***
Flavin coenzymes participate in 2 electron oxidoreduction reactions that can proceed one or two
electrons at a time.
Flavin reduction proceeds through a stable semiquinone free radical species. FAD can transfer 1
electron at a time; NADH and NAD+ can transfer 2 electrons in the form of a hydride, but transfers 2
electrons in 1 step.
The flavins can transfer 1 or 2 electrons, so 1 or 2 steps. E3 uses a stable semiquinone form and is a 2
step process.
Forms:
1. Oxidizied
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Circular pathway citric acid cycle (tca cycle/krebs cycle) is the central aerobic pathway for oxidizing all metabolic fuels. Entry to this pathway is acetyl coa and pyruvic acid. Entry of fuel as pyruvate from glycolysis into the citric acid cycle. Most of the energy yield from the oxidation of substrates in the citric acid cycle is stored in reduced electron carries such as nadh and fadh2. In stage 1. 5, the carbon from metabolic fuels is incorporated into acetyl coa via pdh. Acetyl coa is what enters the citric acid cycle: stage 1 = glycolysis. In stage 2, the citric acid cycle oxidizes actyl-coa to product co2, reduced electron carriers, and a small amount of atp. In stage 3, the reduced electron carriers are re-oxidized via the etc providing energy for synthesis of additional atp. Stages 1. 5 and 2 of respiration occur in the mitochondrial matrix.
