Exam 3 Flashcards
In contrast to glycolysis, the pentose phosphate pathway allows the complete oxidation of glucose to CO2.
True.
When this occurs, the overall stoichiometry involves the input of 6 molecules of glucose-6-phosphate with the production of 12 NADPH, 6 CO2, and the regeneration of 5 molecules of glucose-6-phosphate.
The formation of DNA and RNA directly depends on high gluconeogenesis activity in the cell.
False.
High activity (especially in rapidly dividing cells) of the pentose phosphate pathway is required to supply ribose-5-phosphate for nucleotide biosynthesis.
Glucose 1-phosphate is the direct product of glycogenolysis.
True.
It is formed by the catalytic action of glycogen phosphorylase on glycogen.
Glycogen synthase catalyzes the linkage of all glucose molecules used in the formation of glycogen in live and muscle tissue.
False.
Glycogen synthase does not catalyze the formation of the alpha (1-6) linkages required for formation of branches. This is accomplished by the enzyme amylo (1,4 - 1,6) transglycosylase (the branching enzyme). In addition, glycogenin catalyzes primer extension.
Glycogen synthase catalyzes the joining of two glucose units, supplied as UDP-glucose, to initiate formation of a glycogen chain.
False.
Glycogen synthase requires an existing polymer of four to eight glucose residues as a primer.
Gluconeogenesis is simply a reversal of glycolysis that occurs when blood glucose levels fall below normal.
False.
While some reactions are reversals of glycolytic reactions, four different enzymes are required to bypass the three metabolically irreversible reactions of glycolysis.
The formation of one mole of glucose by gluconeogenesis from pyruvate requires the same amount of energy as that produced by glycolytic degradation of one mole of glucose to pyruvate.
False.
The conversion of two moles of pyruvate to one mole of glucose requires 6 moles of ATP. ATP or equivalent is required for the conversions of:
(a) pyruvate to oxaloacetate
(b) oxaloacetate to phosphoenolpyruvate
(c) 3-phosphoglycerate to 1,3-bisphosphoglycerate.
Each of these reactions is used twice to convert two pyruvates to one glucose.
The Cori cycle is a combination of glycolysis and gluconeogenesis occurring in different body tissues.
True.
It involves glycolysis in peripheral tissues and gluconeogenesis in the liver.
The enzymes that catalyze the reactions of the pentose phosphate pathway are all found in the cytosol.
True.
Since the pentose phosphate pathway works in conjunction with glycolysis, the most logical place for it to be located is in the cytosol.
The enzymes phosphfructokinase-1 and fructose 1,6-bisphosphatase form a substrate cycle.
True.
The interconversion of fructose-6-phosphate and fructose 1,6-bisphosphate in glycolysis and gluconeogenesis comprise a substrate cycle. Control over both of these enzymes is extremely important to prevent the wasteful use of energy in the cell.
Normally, the brain relies almost entirely on glucose for its energy needs.
True.
Hence, normal blood glucose levels are maintained at all times, even at the expense of peripheral (muscle) tissue.
Note: The brain does utilize ketone bodies during starvation.
Most glucose 6-phosphate produced in the liver from glycogenolysis is converted to free glucose for delivery to cells of other tissues.
True.
The glucose is delivered to brain cells, adipocytes, and erythrocytes.
Glucagon is a small peptide hormone that stimulates glycogenolysis by specifically targeting liver cells.
True.
Only liver cells are rich in glucagon receptors, thus the liver is targeted by glucagon.
In the liver, glucagon causes the activation of phosphofructokinase-2 which produced fructose 2,6-bisphosphate, an activator of phosphofructokinase-1 and inhibitor of fructose 1,6-bisphosphatase.
False.
Insulin would cause the activation of phosphofructokinase-2. Glucagon causes the activation of fructose 2,6-bisphosphate which removes fructose 2,6-bisphosphate, thus activating fructose 1,6-bisphosphatase and inhibiting phosphofructokinase-1.
The coenzyme required for reductive biosynthesis (e.g., of fatty acids), that is produced by the pentose phosphate pathway, is _________ .
NADPH
A pentose phosphate pathway enzyme that catalyzes the transfer of a three-carbon unit from a ketose-phosphate to an aldose-phosphate is called a/an __________ .
transaldolase
Name the pathway or process discussed in text Chapter 12 to which each of the following belongs:
pyruvate carboxylase _______________
ribose 5-phosphate _______________
glucose 6-phosphatase _______________
UDP-glucose _______________
gluconeogenesis
pentose phosphate pathway
gluconeogenesis or pentose phosphate pathway
glycogen synthesis
Glycogen phosphorylase catalyzes the degradation of glycogen chains from their nonreducing ends, but stops four glucose residues from a branch point. The remaining molecule is called a/an _____________ .
limit dextrin
Gluconeogenesis requires four enzymes that are not enzymes of the glycolytic pathway. These four enzymes are _____________, _____________, _____________, and _____________ .
pyruvate carboxylase
phosphoenolpyruvate carboxy kinase
fructose 1,6 bisphosphatase
glucose 6-phosphatase
The protein that is attached to the glycogen primer required for glycogen synthesis is called ____________ .
glycogenin
In mammals, the hormone _____________ activates glycogen degradation in the muscles while ___________ activates glycogen synthesis.
epinephrine; insulin
PEP carboxykinase is regulated by ______________ of the gene in response to increased cAMP levels that result from prolonged release of glucagon.
hormonal induction
Three major gluconeogenic precursors are ___________, which comes from anaerobic muscle, ____________, which comes from the breakdown of protein in peripheral tissue, and ___________, which comes from metabolism of triacylglycerols.
lactate
alalnine (and most other amino acids)
glycerol
A pair of reactions that both form and degrade a specific substrate, in order to fine tune regulation of metabolism, is called a ____________ .
substrate cycle
The primary sites of gluconeogenesis in mammals is the _________, although small amounts activity occur in _________ and _________ .
liver
kidney
small intestine
FAD is used in the citric acid cycle as an oxidizing agent.
True.
FAD is a prosthetic group of an enzyme of the succinate dehydrogenase complex that accepts a hydride and a proton from succinate and immediately transfers them to coenzyme Q, the mobile carrier of reducing power within the inner mitochondrial membrane.
The final steps in the oxidation of glucose to CO2 occur in the citric acid cycle.
True.
Six molecules of CO2 are produced in the mitochondria for each molecule of glucose catabolized. Two molecules of CO2 are produced by the action of pyruvate dehydrogenase and four molecules of CO2 are produced within the citric acid cycle.
High concentrations of all intermediates of the citric acid cycle must be present for its efficient operation.
False.
Your text points out that the cycle is a multistep catalyst. Since the process is cyclic, the intermediates are continually recycled. However, sufficient amounts of oxaloacetate must be present to allow appropriate rates of acetyl CoA uptake. However, the cycle is extremely efficient, even with very low concentrations of oxaloacetate.
There are more six-carbon intermediates in the citric acid cycle than four-carbon intermediates.
False.
There are two six-carbon intermediates, one five-carbon intermediate, and five four-carbon intermediates.
In eukaryotes, the enzymes that catalyze the reactions of the citric acid cycle are found in the cytosol.
False.
They are located in mitochondria.
Pyruvate is produced in the eukaryotic cytosol but enters the mitochondrial matrix unaided.
False.
Pyruvate does pass freely through porins in the outer mitchondrial membrane, but its passage, in symport with H+, through the inner mitochondrial membrane is aided by pyruvate translocase.
The eukaryotic pyruvate dehydrogenase complex is the largest multienzyme complex known.
True.
It is several times larger than a ribosome.
Intermediates of the citric acid cycle that are used for anabolic (synthetic) needs of the cell are replenished by anaplerotic reactions.
True.
Any time an intermediate is removed from the citric acid cycle, it must be replenished by the input of another intermediate of the cycle. Two of the most common inputs are oxaloacetate and alpha-ketoglutarate.
Porphyrin biosynthesis could potentially interfere with the citric acid cycle by depleting succinyl CoA.
True.
Succinyl CoA condenses with glycine to initiate porphyrin biosynthesis, but succinyl CoA can be replenished by degradation of some amino acids. Additionally, replacement with any other intermediate of the cycle will fill the void created by the loss of succinyl CoA.
Two moles of coenzyme A are consumed per mole of pyruvate that enters the mitochondria, one in the production of acetyl CoA and the other in the production of succinyl CoA.
False.
Although coenzyme A is required in two different steps as pyruvate is catabolized via the citric acid cycle, two moles of coenzyme A are formed as products, one by citrate synthase and the other by succinyl CoA synthetase, so there is not net consumption of coenzyme A.
The citric acid cycle is essentially controlled entirely by the availability of acetyl CoA and cycle intermediates.
False.
In addition to the availability of acetyl CoA and oxaloacetate to initiate citrate formation, control is exercised through allosteric modulators and their effects on the cycle enzymes citrate synthase, isocitrate dehydrogenase, and the alpha-ketoglutarate dehydrogenase complex.
Per mole of glucose, the citric acid cycle produces as many high energy phosphate molecules by substrate level phosphorylation as does glycolytic catabolism of glucose to pyruvate.
True.
Glycolysis produces two ATP per mole of glucose. From the two moles of pyruvate produced by glycolysis, the citric acid cycle produces two moles of GTO which get converted to ATP.
The two citric acid cycle oxidative steps that are skipped during the glyoxylate pathway are alpha ketoglutarate dehydrogenase and succinate dehydrogenase.
False.
The glyoxylate cycle skips isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. The two oxidative steps that are skipped are those that produce CO2 in the citric acid cycle.
The alpha ketoglutarate dehydrogenase complex is very much like the pyruvate dehydrogenase complex.
True.
Both are complexes of several different enzymes and utilize the same cofactors (TPP, lipoamide, FAD, CoASH, NAD+)
The reaction catalyzed by the succinate dehydrogenase complex is the only step in the citric acid cycle that involves substrate level phosphorylation.
False.
It is the step catalyzed by succinyl CoA synthase that results in substrate level phosphorylation as GTP (or ATP) is formed.
The oxidation of pyruvate to acetyl CoA and CO2 requires the cofactors __________, __________, ____________, and ___________.
coenzyme A
NAD
lipoate, FAD
TPP
The oxidation of succinate utilizes the enzyme-bound coenzyme ___________ due to the need for a stronger oxidizing agent that NAD.
FAD
The conversion of isocitrate to alpha ketoglutarate also produces ___________ . This is an oxidative process in which the actual oxidant is __________ .
CO2
NAD
