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where does the citric acid cycle take place in the cell?
the activated carrier of acyl groups is:
in the presence of oxygen, pyruvate is converted to:
a large, multi-subunit enzyme complex that links glycolysis and the citric acid cycle under aerobic conditions:
provides a flexible linkage between active sites within the pyruvate dehydrogenase (PDH) complex:
pyruvate is a decarboxylated at this active site in PDH:
the coenzyme _____ is the prosthetic group in the decarboxylation of pyruvate:
arsenite inhibits the function of this component of the PDH:
the type of enzyme regulation process that inhibits the PDH complex:
caused by a deficiency in vitamin B1:
the citric acid cycle is also known as the:
- Krebs cycle
- Cori cycle
acetyl CoA is:
- the activated form of acyl groups
- the fuel for the citric acid cycle
pyruvate is decarboxylated by which subunit of the PDH?
which enzyme(s) is/are responsible for the following reaction...
pyruvate + CoA + NAD+ > acetyl CoA + NADH + H+ + CO2
pyruvate dehydrogenase complex
what are the steps involved (in order) in the conversion of pyruvate to acetyl CoA?
3) transfer to CoA
some conditions that will activate PDH kinase, which catalyzes the phosphorylation and inactivation of E1 in the PDH complex:
elevated concentrations of NADH and ATP
in addition to PDH, some other enzymes that are key regulatory sites in the citric acid cycle:
- isocitrate dehydrogenase
- alpha-ketoglutarate dehydrogenase
- citrate synthase
how are the three active sites of PDH linked?
the long, flexible arm of the E2 subunit acts like a long robotic arm and carries the substrate from active site to active site
what reaction serves to link glycolysis and the citric acid cycle?
pyruvate + CoA + NAD+ > acetyl CoA + NADH + H+ + CO2
the key means of regulation of PDH:
the inactive form of PDH is the phosphorylated form of the E1 component - PDH phosphatase activates PDH by removing the phosphoryl group and PDH kinase phosphorylates and inactivates it again when the energy charge is high
if NADH levels are high, what is the fate of acetyl CoA?
the PDH complex is inhibited by high levels of NADH, so glucose is spared and acetyl CoA is used in the synthesis of fatty acids
in the absence of oxygen (anaerobic conditions), pyruvate is converted into:
- lactic acid
in the presence of oxygen (aerobic conditions), pyruvate is converted into:
a molecule called acetyl coenzyme A (acetyl CoA), that is able to enter the citric acid cycle (this takes place in the mitochondria)
in most tissues, how is pyruvate processed?
aerobically because oxygen is readily available
pyruvate is converted into acetyl CoA usually in:
resting muscle activity in higher organisms
glycolysis takes place in what part of the cell?
in the cytoplasm of the cell
the citric acid cycle takes place in what part of the cell?
in the mitochondria
where must pyruvate be transported to be aerobically metabolized?
into the mitochondria
in the mitochondrial matrix, pyruvate is oxidatively decarboxylated by:
the pyruvate dehydrogenase complex to form acetyl CoA
the link between glycolysis and the citric acid cycle:
the irreversible conversion of pyruvate into acetyl CoA
glucose can be metabolized in glycolysis to:
- 2 pyruvate
- 2 ATP
why is glycolysis inefficient?
it captures only a fraction of the energy inherent in a glucose molecule as ATP
more of the energy can be accessed if pyruvate...
is completely oxidized into CO2 and H2O
cellular respiration in eukaryotes...
takes place inside the mitochondria and can be divided into two parts
the citric acid cycle is also known as:
- the tricarboxylic acid cycle
- the Krebs cycle
what does the pyruvate dehydrogenase complex produce and capture?
it produces CO2 and captures high-transfer-potential electrons in the form of NADH
some characteristics of the pyruvate dehydrogenase complex:
it is a large, highly integrated complex of three distinct enzymes, each with its own active site
the two parts of cellular respiration in eukaryotes (take place in the mitochondria):
1) the citric acid cycle (aka, the Krebs cycle)
2) the electron transport chain
carbon fuels are completely oxidized with a concomitant generation of high-transfer-potential electrons:
the citric acid cycle
the citric acid cycle processes:
the two-carbon acetyl unit to 2 molecules of CO2 while generating high-energy electrons that can be used to form ATP
the high-transfer-potential electrons produced in the citric acid cycle are transferred to oxygen to form water in a series of oxidation-reduction reactions:
the electron transport chain
the electron transport chain is highly...
exergonic, and the released energy is used to synthesize ATP (oxidative phosphorylation)
bond in acetyl CoA:
the thioster bond is a very high energy bond, and therefore unstable - the acetyl group can be easily transferred to other molecules
pyruvate dehydrogenase forms:
acetyl CoA from pyruvate
pyruvate dehydrogenase links glycolysis with cellular respiration by:
irreversibly converting pyruvate into acetyl CoA
a) glycolysis takes place in which part of the cell?
b) the citric acid cycle takes lace where?
a) glycolysis - cytoplasm
b) citric acid cycle - mitochondria
the synthesis of acetyl CoA from pyruvate requires:
3 enzymes and 5 coenzymes
what are the 5 coenzymes?
1) thiamine pyrophosphate (TPP) - E1
2) lipoic acid - E2
3) flavin adenine dinucleotide (FAD) - E3
5) nicotinamide adenine dinucleotide (NAD+)
coenzymes that are not permanently altered by participation in the reaction:
coenzymes that function as substrates:
a thiamine derivative which functions as a cofactor for pyruvate dehyrodgenase component (E1), and catalyzes the reversible decarboxylation reaction:
cofactor for transacetylase component (E2):
lipoid acid is cycled between:
its reduced and oxidized form, and acetylated and de-acetylated form
the conversion of pyruvate into acetyl CoA consists of 3 steps:
3) the transfer of the resultant acetyl group to CoA
pyruvate combines with the ionized form of TPP and is then decarboxylated to yield hydroxyethyl-TPP:
the decarboxylation portion of the reaction is catalyzed by:
pyruvate dehydrogenase component (E1) of the multi enzyme complex
what is the coenzyme of the pyruvate dehydrogenase component?
the hydroxyethyl group attached to TPP is oxidized to form an acetyl group while being simultaneously transferred to lipoamide, a derivative of lipoic acid:
the oxidation portion of the reaction results in:
the formation of an energy-rich thioester bond
the oxidation portion of the reaction is also catalyzed by the pyruvate dehydrogenase component E1, and yields:
the acetyl group is transferred from acetyllipoamide to CoA to form acetyl CoA:
formation of acetyl CoA
the formation of acetyl CoA is catalyzed by:
dihydrolipoyl transacetylase (E2)
the oxidized form of lipoamide is regenerated by:
dihydrolipoyl dehdyrogenase (E3)
regulation of pyruvate dehydrogenase complex is very complex - there are 2 regulatory mechanisms:
1) allosteric regulation
2) covalent regulation
are we able to convert acetyl CoA back into glucose?
the oxidative decarboxylation of pyruvate to acetyl CoA commits the carbon atoms of glucose to either two principal fates:
1) oxidation to CO2 by the citric acid cycle with the concomitant generation of energy
2) incorporation into lipid, because acetyl CoA is a key precursor for lipid synthesis
high concentrations of NADH and acetyl CoA inform the enzyme that:
the energy needs of the cell have been met
NADH and acetyl CoA are products of the PDC complex, and they are both:
negative allosteric effects of PDC
PDC is turned on again when the energy demand on the cell is high:
low NADH and low acetyl CoA
covalent regulation by:
reversible phosphorylation of E1
phosphorylation of the pyruvate dehydrogenase component (E1) by:
a pyruvate dehydrogenase (PDH) kinase switches off the activity of the complex
deactivation is reversed by the action of:
a pyruvate dehydrogenase phosphatase
two highly regulated enzymes:
PDH kinase and PDH phosphatase
pyruvate dehydrogenase is switched off when:
the energy charge is high
PDP is stimulated by:
Ca2, and thus Ca activates PDC
a neurological condition that results from a deficiency of thiamin:
catalyzes the link between glycolysis and the citric acid cycle:
pyruvate dehydrogenase complex
coenzyme required by transacetylase:
final product of pyruvate dehydrogenase:
catalyzes the formation of acetyl CoA:
regenerates active transacetylase:
fuel for the citric acid cycle:
coenzyme required by pyruvate dehdyrogenase:
catalyzes the oxidative decarboxylation of pyruvate:
central metabolic hub:
citric acid cycle
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