bich 409 exam four/final exam study set

what is gluconeogenesis?

the synthesis of glucose from noncarbohydrate precursors

what is the primary energy source for the brain?

glucose

when must glucose be synthesized from noncarbohydrate precursors in order to maintain the blood glucose levels?

for fasting periods longer than one day, or during periods of intense exercise

what are the noncarbohydrate precursors used in gluconeogenesis?

- pyruvate

- lactate

- oxaloacetate

- gluconeogenic amino acids

- glycerol

in what forms do the noncarbohydrate precursors enter the gluconeogenic pathway?

- pyruvate

- oxaloactate

- dihydroxyactetone phosphate

what are the two major sites for gluconeogenesis?

the liver (90%) and the kidneys (10%)

what are the three irreversible steps of glycolysis catalyzed by, which are the steps that have to be bypassed in gluconeogenesis?

- hexokinase

- phosphofructokinase

- pyruvate kinase

how many NTPs does gluconeogenesis consume in order to make it a spontaneous (exergonic) process?

six

what are the two steps required to convert pyruvate back into phosphoenolpyruvate (PEP) during gluconeogenesis?

1. pyruvate carboxylase (CO2 + ATP + H2O --> ADP + Pi + 2H+): carboxylation of pyruvate to form oxaloacetate

2. PEP carboxykinase (GTP --> GDP + CO2): oxaloacetate is decarboxylated and phosphorylated to produce phosphoenolpyruvate

what enzyme aids the process to overcome the generation of fructose-6-phosphate from fructose-1,6-bisphosphate?

fructose-1,6-bisphosphatase

what is the enzyme that overcomes the irreversible step catalyzed by hexokinase which is the phosphorylation reaction converting glucose-6-phosphate into glucose?

glucose-6-phosphatase

where is glucose-6-phosphatase found?

- the lumen of the endoplasmic reticulum

- not present in the muscles or the brain so glucose cannot be produced by these organs

where does pyruvate carboxylase get its energy and what does it do?

it uses the energy of ATP to carboxylate pyruvate to form oxaloacetate

what does the enzyme pyruvate carboxylase require?

biotin, a cofactor that is covalently attached to a lysine residue

what does biotin carry?

it serves as a carrier of activated carbon dioxide

what is the first step of pyruvate carboxylase?

to activate a molecule of bicarbonate for nucleophilic attack

how is bicarbonate activated?

- by phosphorylation with ATP to produce a high energy carboxyphosphate intermediate

what is the second step in the reaction of pyruvate carboxylase?

carboxyphosphate then undergoes nucleophilic attack by the N1 atom of biotin to form N-carboxybiotin

what is the third step in the reaction of pyruvate carboxylase?

to remove a proton from the C3 of pyruvate to generate a resonance stabilized carbanion which is a good nucleophile, attacking the N-carboxybiotin to regenerate biotin and form oxaloacetate

what allosterically activates pyruvate carboxylase?

acetyl-CoA

what is necessary of acetyl-CoA in order to activate bicarbonate?

acetyl-CoA must be bound to an allosteric binding site of the enzyme

what is the second half of the pyruvate carboxylase catalyzed reaction? is his affected by the allosteric regulator acetyl-Co-A?

- the nucleophilic attack of the pyruvate enolate on N-carboxybiotin

- no

what is the physiological regulation of the activation of bicarbonate by acetyl-CoA?

- if the concentration of ATP is low (ATP is a substrate for pyruvate carboxylase) and/or the concentration of acetyl-CoA is low, then pyruvate is directed into the citric acid cycle to promote the synthesis of ATP

- if the concentration of ATP and acetyl-CoA are high, pyruvate is converted into oxaloacetate and consumed in gluconeogenesis

- high concentrrations of ATP and acetyl-CoA are signals that the cell's energy level is high and metabolites are converted into glucose

- if the energy status of the cell is low, the concentrations of ATP and acyl-CoAs are low and pyruvate is directed towards the TCA cycle (citric acid).

where is the only place pyruvate carboxylase is found?

in the matrix of the mitochondria

following pyruvate carboxylase, what is the next enzyme involved in gluconeogenesis?

PEP carboxykinase (phosphoenolpyruvate carboxykinase)

what are the two places PEP carboxykinase is found?

in both the mitochondria or in the cytosol

(depends on the organism and the tissue)

where is PEP carboxykinase found in human livers?

in both the mitochondria and the cytosol

where is pyruvate transported and what is it converted into once it gets there?

- transported into the mitochondria

- converted into acetyl-CoA by pyruvate dehydrogenase --> citric acid cycle OR

- converted into oxaloacetate by pyruvate carboxylase and used for gluconeogenesis

which enzyme converts pyruvate into acetyl-CoA for use in the citric acid cycle?

pyruvate dehydrogenase

which enzyme converts pyruvate into oxaloacetate for use in gluconeogenesis?

pyruvate carboxylase

what happens in tissues where PEP carboxykinase is found only in the mitochondria? where can this enzyme be transported?

- oxaloacetate is converted into PEP

- can be transported to the cytosol

what happens in tissues where PEP carboxykinase is found only in the cytosol?

oxaloacetate must be reduced to malate so that it can be transported across the mitochondrial membrane to the cytosol where the malate is reoxidized into oxaloacetate and goes on through gluconeogenesis

what two things does the enzyme PEP carboxykinase do simultaneously?

decarboxylates and phosphorylates oxaloacetate

what happens to the CO2 in the mechanism of PEP carboxykinase?

- CO2 is added in the pyruvate carboxylase step

- this CO2 comes off in the PEP carboxykinase step

is the decarboxylation in the mechanism of PEP carboxykinase endergonic or exergonic? what does this process help to drive?

- it is very exergonic

- helps to drive the otherwise highly endergonic reaction of PEP synthesis

what happens after PEP is formed by PEP carboxykinase?

- the PEP is hydrated by enolase to form 2-phosphoglycerate which is in turn isomerized into 3-phosphoglycerate by phosphoglycerate mutase

- phosphoglycerokinase uses ATP to produce 1,3-bisphosphoglycerte which is reduced by GAPDH into glyceraldehyde-3-phosphate

which enzyme produces dihydroxyacetone phosphate?

triose phosphate isomerase

which enzyme catalyzes an aldol condensation of DHAP and glyceraldehyde-3-phosphate to form fructose-1,6-bisphosphate?

aldolase

which enzymes function in both glycolysis and gluconeogenesis, and why?

- PEP carboxykinase

- enolase

- phosphoglycerate mutase

- phosphoglycerokinase

- triose phosphate isomerase

- aldolase

- because they are all near equilibrium and reversible

what is the first committed step of glycolysis and what enzyme catalyzes it? is it reversible or irreversible?

- phosphofructokinase

- the phosphorylation of fructose-6-phosphate by phosphofructokinase

- irreversible

in gluconeogenesis, which enzyme hydrolyzes fructose-1,6-bisphosphate? what type of regulation is this enzyme?

- fructose-1,6-bisphosphatase

- allosterically regulated enzyme

the reaction of the phosphorylation and/or hydrolyzation of fructose-1,6-bisphosphate is (exergonic/endergonic), (reversible/irreversible), and (non-regulated/regulated).

- exergonic

- irreversible

- regulated

does the reaction involving fructose-1,6-bisphosphate increase, decrease, or have no effect on the following:

- citrate

- fructose-2,6-bisphosphate

- AMP

- citrate: increase

- fructose-2,6-bisphosphate: decrease

- AMP: decrease

what effect does AMP have on fructose-2,6-bisphosphate?

it enhances the inhibition of fructose-2,6-BP

what enzyme is involved in the step following the conversion of fructose-1,6-BP to fructose-1-P? what is the result of this reaction?

- phosphoglucose isomerase

- isomerizes fructose-1-P into glucose-6-P

what is the final step of gluconeogenesis?what enzymes catalyzes this final reaction?

- the conversion of glucose-6-P to glucose

- glucose-6-phosphatase

what type of protein is the enzyme glucose-6-phosphatase? where is this protein found? what types of tissues is this enzyme absent in?

- membrane bound protein

- found in the membranes of the endoplasmic reticulum of the liver and kidneys

- absent in tissues such as the brain and muscle cells

how is the association of glucose-6-phosphatase with the membrane important to its function?

- because the substrate glucose-6-P is hydrolyzed as it passes through the membrane into the endoplasmic reticulum

- vesicles form from the ER membrane enclosing the molecules of glucose. the vesicles then diffuse to the plasma membrane and fuse with it

- when the vesicle fuses with the plasma membrane, all of the glucose contained within the vesicle is released into the bloodstream

how does vigorous exercise affect the muscle cells?

leads to an oxygen shortage producing anaerobic conditions in the muscle cells

what happens in order to regenerate NAD+ and keep glycolysis going?

pyruvate is reduced to lactate

what happens to the lactate produced by the muscles?

- it is released into the blood where it is carried to the liver

- in the liver, the lactate is reoxidized into pyruvate which is then converted to glucose via gluconeogenesis

- in this way, the liver shares in the the metabolic stress produced by vigorous exercise

what happens in the liver and muscles during exercise? what is the name for this cycle between the liver and muscles?

- it releases glucose into the blood stream to fuel the muscles

- the muscles produce lactate which is carried back to the liver whereby gluconeogenesis it is converted back into glucose

- the Cori cycle

where do all of the reactions of glycolysis and most of the reactions of gluconeogenesis occur?

in the cytosol

in what way are nearly all of the reactions of glycolysis and gluconeogenesis regulated?

reciprocally regulated

what is the result of reciprocal regulation?

it allows the cell to rapidly catabolize glucose when the cellular energy supply is low and to synthesize and store glucose when the energy level of the cell is high

which enzymes catalyze the three irreversible steps of glycolysis? how are this enzymes regulated?

- hexokinase, phosphofructokinase, and pyruvate kinase

- allosterically regulated

what does gluconeogenesis do combat these irreversible reactions? which enzymes catalyze these reactions?

- replaces these irreversible reactions with corresponding irreversible reactions that are exergonic in the direction of glucose synthesis

- glucose-6-phosphatase, fructose-1,6-bisphosphatase, and the sequential reactions of pyruvate carboxylase and PEP carboxykinase

how is glucose-6-phosphatase regulated?

- it is not under allosteric control

- the Km of glucose-6-phosphatase for glucose-6-P is considerably higher than the cellular concentrations of glucose-6-P

- as a result, glucose-6-phosphatase displays a linear dependence of activity as a function of glucose-6-P concentration

- this enzyme is said to be under substrate-level control

what molecule is an allosteric effector of both glycolysis and gluconeogenesis?

acetyl-CoA

what enzyme does acetyl-CoA inhibit?

pyruvate kinase

what enzyme does acetyl-CoA reciprocally activate?

pyruvate carboxylase

what enzyme does acetyl-CoA inactivate? what does this inactivation provide?

- pyruvate dehydrogenase

- provides a regulatory link between glycolysis and the citric acid cycle

what are high concentrations of acetyl-CoA indicative of?

high energy supplies

what happens between metabolites and glycogen when the energy supply is high?

metabolites are directed towards storage in the form of glycogen

fructose-1,6-bisphosphatase is another important site of ________________ regulation.

gluconeogenesis

high concentrations of what activates fructose-1,6-bisphosphatase?

high concentrations of citrate

high concentrations of what inhibits fructose-1,6-bisphosphatase?

AMP

what molecule is an important allosteric regulator of fructose-1,6-bisphosphatase?

fructose-2,6-bisphosphate

what does fructose-2,6-BP do to fructose-1,6-bisphosphatase?

it inactivates fructose-1,6-bisphosphatase

do the concentrations of the following increase, decrease, or remain the same in the presence of fructose-1,6-bisphosphatase?

- citrate: increases

- fructose-2,6-BP: decreases

- AMP: decreases

what molecule enhances the inhibition of fructose-2,6-BP?

AMP

what is an important note about the allosteric effectors of fructose-1,6-bisphosphatase compared to phosphofructokinase? how do these effectors regulate both enzymes?

- the allosteric effectors of fructose-1,6-BPase are all allosteric effectors of phosphofructokinase

- they reciprocally regulate both enzymes

this provides a way for gluconeogenesis and glycolysis to be coordinated such that when one pathway is ____________ the other pathway is _______________.

- active

- inactive

what would happen if both gluconeogenesis and glycolysis were active at the same time?

- the net result would be the net hydrolysis of 2ATPs and 2GTPs per reaction cycle

- both pathways are highly exergonic so there is no thermodynamic barrier to such cycling

what type of cycle is the cycling of fructose-6-P and fructose-1,6-BP?

a substrate cycle

what is the net result of the substrate cycling if the flux of metabolites through each pathway is the same?

the net result of the substrate cycling would be the net hydrolysis of ATP

regardless of which pathway is activated (the other pathway will be reciprocally __________) there is always flux of _____________ through both pathways.

- inactivated

- metabolites

how is glucose catabolized in most animal tissues?

- it is catabolized via the glycolytic pathway into two molecules of pyruvate

- pyruvate is then oxidized via the citric acid cycle to generate ATP

what is an alternate metabolic fate for glucose used to generate NADPH and specialized products needed by the cell?

- the pentose phosphate pathway

(aka the hexose monophosphate shunt or phosphogluconate pathway)

what is produced by the pentose phosphate pathway? what is this molecule?

- NADPH

- it's the universal reductant in anabolic pathways

in mammals, which types of tissues are those requiring large amounts of NADPH produced by the pentose phosphate pathway?

the tissues that synthesize fatty acids and steroids such as the mammary glands, adipose tissue, adrenal cortex, and the liver

which types of tissues are virtually lacking the pentose phosphate pathway?

those less active in fatty acid synthesis such as skeletal muscle

what is a second function of the pentose phosphate pathway?

the generation of pentoses, particularly ribose, which is necessary for the synthesis of nucleic acids

what are the two phases of the pentose phosphate pathway?

1. the oxidative phase

2. the nonoxidative phase

what occurs in the first phase of the pentose phosphate pathway, the oxidative phase? briefly describe the steps.

- two of the first three reactions of the first phase generate NADPH

briefly describe the steps of the pentose phosphate pathway.

1. glucose-6-P is oxidized into ribulose-5-P and CO2 (NADP+ is reduced into NADPH)

2. ribulose-5-P is converted into other pentose-5-phosphates including ribose-5-P used to synthesize the nucleic acids

3. includes a series of reactions that convert three of the pentose-5-phosphates into two molecules of hexoses and one triose

4. some of these sugars are converted into glucose-6-P so the cycle can be repeated

how does the direction of the pentose phosphate pathway vary?

it varies to meet different metabolic conditions

how many steps make up the oxidative phase of the pentose phosphate pathway? what are these steps and the enzymes that catalyze them?

- three

1. glucose-6-P --> 6-phosphoglucolactone (glucose-6-P dehydrogenase -- NADP+ --> NADPH + H+)

2. 6-phosphoglucolactone --> 6-phosphogluconate (glucolactonase -- H2O --> H+)

3. 6-phosphogluconate --> ribulose-5-P (6-phosphogluconate dehydrogenase -- NADP+ --> NADPH + H+)

how many steps and reactions make up the nonoxidative phase of the pentose phosphate pathway?

- 5 steps

- 4 types of reactions

briefly describe the final 5 steps of the pentose phosphate pathway and the enzymes that catalyze them.

4. phosphopentose isomerase (ribulose-5-P <--> enediol intermediate <--> ribose-5-P)

5. phosphopentose epimerase (ribulose-5-P <--> enediolate intermediate <--> xylulose-5-P)

6. transketolase (xylulose-5-P + ribose-5-P <--> glyceraldehyde-3-P + sedoheptulose-7-P)

7. transaldolase (glyceraldehyde-3-P + sedoheptulose-7-P <--> fructose-6-P + erythrose-4-P)

8. another job for transketolase (erythrose-4-P + xylulose-5-P --> fructose-6-P + glyceraldehyde-3-P)

what cofactor is required for the functioning of transketolase?

thiamine pyrophosphate

what is required for the functioning of transaldolase?

Schiff base formation

what is the sum of the oxidative and nonoxidative phases in the pentose phosphate pathway?

3 glucose-6-P + 6 NADP+ --> 2 fructose-6-P + glyceraldehyde-3-P + 3CO2 + 6 NADPH + 6 H+

can the pentose phosphate pathway be tailored to meet specific needs of the cell?

yes

how is the pentose phosphate pathway tailored for cells that require both ribose-5-P and NADPH?

glucose-6-P -(lose NADPH)-> 6-phosphogluconate -(lose NADPH +CO2)-> ribulose-5-P --> ribose-5-P

how is the pentose phosphate pathway tailored for cells that need more ribose-5-P than NADPH?

(see pathway on page 8 of pentose phosphate pathway prof notes)

how is the pentose phosphate pathway tailored for cells that need more NADPH than ribose-5-P?

(see pathway on page 8 of pentose phosphate pathway prof notes)

how is the pentose phosphate pathway tailored for cells that need both NADPH and ATP but not ribose-5-P?

(see pathway on page 9 of pentose phosphate pathway prof notes)

what enzyme catalyzes the reaction of the first step of the pentose phosphate pathway? what kind of step is this? how is this step regulated? what is the product of this reaction?

- glucose-6-P dehydrogenase

- irreversible and committed step

- allosterically regulated

- NADPH

what type of inhibitor is NADPH? what does the cytosol concentration of NADPH mean for the activity of the enzyme glucose-6-P dehydrogenase? how is NADPH regulated and what regulates it? what type of regulation is the transcription of the gene for this enzyme under?

- a strong inhibitor

- high cytosol concentration of NADPH means the enzyme's activity is low

- allosterically regulated by fatty acid acyl esters of coenzyme A

- hormonal regulation