The citric acid cycle utilizes mitochondrial enzymes. The first step is fusion of the acetyl group of acetyl-CoA with oxaloacetate, catalyzed by
citrate synthase
. CoA-SH and heat are released and citrate is produced. Citrate is isomerized by dehydration and rehydration to isocitrate.
What is regenerated in the TCA cycle?
In the reaction
NADPH
is consumed, which is proposed to be regenerated by the tricarboxylic acid (TCA) cycle enzymes present in whole cells or crude cell extract. Citrate is isomerized to isocitrate and then further converted by the enzyme isocitrate dehydrogenase (IDH). In this process IDH regenerates NADPH.
What is the role of oxaloacetate in the citric acid cycle?
Oxaloacetate is
an intermediate of the citric acid cycle, where it reacts with acetyl-CoA to form citrate
, catalyzed by citrate synthase. It is also involved in gluconeogenesis, the urea cycle, the glyoxylate cycle, amino acid synthesis, and fatty acid synthesis.
Why is it important for a cell to regenerate oxaloacetate?
Why is it important for a cell to regenerate oxaloacetate? Oxaloacetate is the first reaction of the Kreb cycle.
If this isn’t regenerated then ATP will not be formed/generated without ATP cells do not function
.
How does oxaloacetate enter gluconeogenesis?
Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate
by the carboxylation of pyruvate
. This reaction also requires one molecule of ATP, and is catalyzed by pyruvate carboxylase.
Why must oxaloacetate be converted to malate in gluconeogenesis?
Because oxaloacetate cannot pass through the mitochondria membranes
it must be first converted into malate by malate dehydrogenase. Malate can then cross the mitochondria membrane into the cytoplasm where it is then converted back into oxaloacetate with another malate dehydrogenase.
What type of reaction is catalyzed by isocitrate dehydrogenase and α ketoglutarate dehydrogenase?
What type of reaction is catalyzed by isocitrate dehydrogenase and α-ketoglutarate dehydrogenase? Both dehydrogenases are involved in
redox reactions
in which a CO2 is abstracted from a substrate. In both cases NAD+ is reduced while the other substrate is oxidized.
Which stage of aerobic respiration produces ATP and releases CO2?
The stage of aerobic respiration which produces ATP and NADH, and releases CO2 is the
Krebs cycle
.
What are the steps of TCA cycle?
- Oxidative Decarboxylation of pyruvate to Acetyl CoA.
- Step 1: Condensation of acetyl CoA with oxaloacetate.
- Step 2: Isomerization of citrate into isocitrate.
- Step 3: Oxidative decarboxylations of isocitrate.
- Step 4: Oxidative decarboxylation of α-ketoglutarate.
What would happen to a cell if a mutation resulted in the formation of insufficient oxaloacetate in the citric acid cycle?
Oxaloacetate is required in the first step of the citric acid cycle, where it combines with acetyl-CoA. This mutation would cause less oxaloacetate to be available to react with acetyl-CoA, and therefore
fewer electron carriers would be reduced and fewer ATP produced
.
What is produced when Succinyl CoA is changed to succinate?
So, the answer is ‘
ATP in plants and GTP in animals
‘.
How is oxaloacetate converted to aspartate?
To complete the shuttle, oxaloacetate must be transported from the matrix to the cytoplasm but there is no such transport system. Instead, oxaloacetate is
first converted to aspartate by aspartate aminotransferase (Chapter 17) and then transported out of the mitochondria via the glutamate-aspartate translocase
.
What would happen if oxaloacetate was not regenerated in the final stage?
If oxaloacetate is removed from the cycle for glucose synthesis, it must be replaced, since if there is not enough oxaloacetate available to form citrate,
the rate of acetyl CoA metabolism, and hence the rate of formation of ATP, will slow down
.
What metabolites can be used to produce oxaloacetate?
The most important anaplerotic pathways are carbon dioxide fixation by either pyruvate carboxylase or PEP carboxylase, leading to the formation of oxaloacetate (Fig. 2.10). Pyruvate carboxylase is activated at a high ATP/ADP ratio and by
acetyl-CoA
and is inhibited by L-aspartate.
Can oxaloacetate be converted to glucose?
In the fasting state,
glucagon causes the liver to mobilize glucose from glycogen (glycogenolysis) and to synthesize glucose from oxaloacetate and glycerol (gluconeogenesis)
. Glucagon stimulates an increase in cyclic adenosine monophosphate leading to an increase in phosphorylation by protein kinase A.
How is oxaloacetate regulated?
The regulation of the supply of oxaloacetate (OAA) for mitochondrial metabolism
via phosphoenolpyruvate carboxylase (PEPC) by metabolites
is studied in barley (Hordeum vulgare L.) leaf protoplasts in light or darkness as well as under photorespiratory or non-photorespiratory conditions.
Can oxaloacetate be converted to pyruvate?
Under gluconeogenic conditions, the TCA cycle intermediates oxaloacetate or malate are converted to pyruvate and PEP by decarboxylation (C4-decarboxylation)
[1, 4] and thus, the PEP–pyruvate–oxaloacetate node provides the direct precursors for gluconeogenesis.
Can oxaloacetate cross mitochondrial membrane?
(1973) and Passarella et al., (1977) already showed that
oxaloacetate does cross inner mitochondrial membranes in counter-transport with another dicarboxylate or Pi
.
How can the cycle continue to function if insufficient oxaloacetate is present?
TRICARBOXYLIC ACID CYCLE
If oxaloacetate is removed from the cycle for glucose synthesis, it must be replaced, since
if there is not enough oxaloacetate available to form citrate, the rate of acetyl CoA metabolism, and hence the rate of formation of ATP, will slow down
.
What type of reaction is catalyzed by isocitrate dehydrogenase and α ketoglutarate dehydrogenase quizlet?
What type of reaction is catalyzed by isocitrate dehydrogenase and α-ketoglutarate dehydrogenase? Both dehydrogenases are involved in
redox reactions
in which a CO2 is abstracted from a substrate. In both cases NAD+ is reduced while the other substrate is oxidized.
How does isocitrate become alpha-ketoglutarate?
This is a two-step process, which involves
oxidation of isocitrate (a secondary alcohol) to oxalosuccinate (a ketone), followed by the decarboxylation of the carboxyl group beta to the ketone
, forming alpha-ketoglutarate.
What reaction does isocitrate dehydrogenase catalyze?
Isocitrate dehydrogenase is a digestive enzyme that is used in the citric acid cycle. Its main function is to catalyze the
oxidative decarboxylation of isocitrate into alpha-ketoglutarate
.
