Biochem Exam 3 Part 6

When must mammals especially maintain sufficient glucose levels?

between meals or after vigorous exercise

How many grams of glucose does the brain require in a day?

120 grams

What is the mount of glucose that can be generated from the body’s glycogen reserves?

190 grams

Why must organisms have a method for synthesizing glucose?

to maintain blood glucose levels within acceptable limits

gluconeogenesis

synthesis of glucose from pyruvate and related 3 and 4 carbon compounds (lactate, amino acids, and glycerol)

Where in the body is the major site of gluconeogenesis?

liver (although can occur in the kidney)

Where in the cell are the major sites of gluconeogenesis?

cytoplasm, mitochondria, ER

Why is gluconeogenesis especially important during fasting or starvation?

because glucose is the primary fuel for the brain and the only fuel for red blood cells

How many pyruvates does gluconeogenesis use to convert into each glucose molecule?

two pyruvates

Pyruvate in the liver via gluconeogenesis can be formed from what and by what enzyme?

• formed from muscle-derived lactate

• enzyme: lactate dehydrogenase

The hydrolysis of what results in glycerol, and what can that product further be converted to?

• triacylglycerol

• dihydroxyacetone phosphate (processed by gluconeogenesis or glycolysis)

The three irreversible steps must be bypassed in gluconeogenesis. What are they?

step 1, step 3, and step 10

Describe this diagram.

This shows the pathway of gluconeogenesis.

Describe this diagram.

This shows the pathway of gluconeogenesis including its unique enzymes.

Describe this diagram.

This shows the individual products and enzymes of gluconeogenesis.

How many reactions are in gluconeogenesis?

Eleven, and the reaction directions are reversed.

What are four enzymes that are unique in gluconeogenesis (glycolysis)?

– Bypass pyruvate kinase: pyruvate carboxylase (mitochondria) and PEP carboxykinase (cytoplasm)

– Bypass Phosphofructokinase: F 1,6,Bisphosphatase (cytoplasm)

– Bypass hexokinase: Glucose-6-phosphatase (ER)

What two enzymes does the formation of phosphoenolpyruvate require? What is the sum of these two reactions?

pyruvate carboxylase and phosphoenolpyruvate carboxylase

What does the conversion of pyruvate into phosphoenolpyruvate begin with the formation of?

oxaloacetate

Where does oxaloacetate formation occur in the cell?

mitochondria

What does CO2 primarily exist as in aqueous solutions?

HCO3-

What vitamin does pyruvate carboxylate require as a cofactor?

biotin

The formation of oxaloacetate by pyruvate carboxylase occurs in how many stages? What are those reactions?

Three stages.

A molecule of ___ is used to power the addition of CO2 to pyruvate.

ATP

Describe this diagram.

This shows the structure of biotin of carboxybiotin. This also shows carboxybiotin covalently bound to ε-amino group of a lysine.

CO2 is attached to the biotinylated enzyme, and then transfer to pyruvate.

biotin

also called vitamin B7, used in CO2 transfer and carboxylation reactions. Biotin deficiency is characterized by muscle pain, lethargy, anorexia, and depression. This vitamin is synthesized by microflora in the intestinal tract and can be obtained in the diet from liver, soybeans, nuts, and many other sources

Oxaloacetate is reduced to ___________ and transported into the _____________, where it is reoxidized to _______________ with the generation of cytoplasmic ______.

malate, cytoplasm, oxaloacetate, NADH

The mitochondrial membrane does not have a transporter for _____________________.

oxaloacetate

What catalyzes the synthesis of PEP from oxaloacetate? Where does this reaction take place?

phospholenolpyruvate carboxykinase (PEPCK) in the cytosol

What happens to the CO2 that was added to pyruvate by pyruvate carboxylase in the decarboxylation reaction?

it comes off

Decarboxylations often drive reactions that are otherwise highly ________________.

endergonic

Describe this diagram.

This shows compartmental cooperation.

Oxaloacetate used in the cytoplasm for gluconeogenesis is formed in the mitochondrial matrix by the carboxylation of pyruvate. Oxaloacetate leaves the mitochondrion by a specific transport system (not shown) in the form of malate, which is reoxidized to oxaloacetate in the cytoplasm.

Reaction uses NADH Malate can then leave the mitochondria and have the reaction reversed by cytosolic malate dehydrogenase.

What is the sum of the pyruvate carboxylase and phosphoenolpyruvate carboxykinase reactions?

This pair of reactions bypasses the irreversible reaction catalyzes by pyruvate kinase in glycolysis.

Phosphoenolpyruvate is metabolized by the enzymes of glycolysis in the reverse direction until…

the next irreversible step, the hydrolysis of fructose 1,6-bisphosphate

What enzyme catalyzes the conversion of fructose 1,6-bisphosphate into fructose 6-phosphate and orthophosphate?

fructose 1,6-bisphosphatase, an allosteric enzyme

The generation of _________________, which occurs essentially only in the ______, is the final step in gluconeogenesis (important control point).

free glucose, liver

Where is glucose 6-phosphate transported into in gluconeogenesis?

lumen of the endoplasmic reticulum

What catalyzes the formation of glucose from glucose 6-phosphate

glucose 6-phosphatase, an integral membrane on the inner surface of the endoplasmic reticulum

Describe this diagram.

This shows the generation of glucose from glucose 6-phosphate.

Several endoplasmic reticulum (ER) proteins play a role in the generation of glucose from glucose 6-phosphate. One transporter brings glucose 6-phosphate into the lumen of the ER, whereas separate transporters carry Pi and glucose back into the cytoplasm. Glucose 6-phosphatase is stabilized by a Ca2+-binding protein.

How many high-energy phosphates are consumed per mole of glucose synthesized by gluconeogenesis?

The equivalent of 11 high-energy phosphates (synthesis of glucose is energetically EXPENSIVE)

What happens when both are running simultaneously?

Futile cycle results ATP/GTP burned, nothing but heat produced

What barrier prevents glycolysis from simply running in reverse to synthesize glucose? How is this barrier overcome in gluconeogenesis?

The reverse of glycolysis is highly endergonic under cellular conditions. The expenditure of six NTP molecules in gluconeogenesis renders gluconeogenesis exergonic.

Gluconeogenesis and glycolysis are regulated so that within a cell, one pathway is relatively ___________ while the other is highly __________.

inactive, active

How is it ensured that gluconeogenesis and glycolysis aren’t highly active at the same time?

the activities of the distinctive enzymes of each pathway are controlled

What is the rationale for reciprocal regulation?

glycolysis will predominate when glucose is abundant and that gluconeogenesis will be highly active when glucose is scarce

The interconversion of fructose 1,6-bisphosphate and fructose 6-phosphate is a key ______________ site.

regulatory

Glycolysis and gluconeogenesis are reciprocally regulated at the interconversion of ________________________ and __________________.

phosphoenolpyruvate, pyruvate

If ATP is needed, does glycolysis or glucogenesis predominate?

glycolysis. Conditions that promote glycolysis inhibit gluconeogenesis.

If glucose is needed, does glycolysis or glucogenesis predominate?

gluconeogenesis. Conditions that promote gluconeogenesis inhibit glycolysis.

Describe this diagram.

This reciprocal regulation of gluconeogenesis and glycolysis in the liver.

What are the rates of glycolysis and gluconeogenesis in the liver adjusted to maintain?

blood-glucose levels

What is the key regulator of glucose metabolism in the liver?

fructose 2,6-bisphosphate

What does fructose 2,6-bisphosphate stimulate and inhibit?

• stimulates phosphofructokinase

• inhibits fructose 1,6-bisphosphate

The kinase that synthesizes ____________________________ (phosphofructokinase 2) and the phosphatase that ________________ this molecule (fructose bisphosphatase 2) are located on the same ________________ chain.

What is this arrangement called?

fructose 2,6-bisphosphate, hydrolyzes, polypeptide

called a bifunctional enzyme

Describe this structure.

This shows the domain structure of the bifunctional regulatory enzyme phosphofructokinase 2/fructose 2,6-bisphosphatase

Which activity of the bifunctional enzyme is determined by blood glucose levels?

When blood glucose is low, the hormone glucagon is secreted (signaled). Then glucagon signaling pathway leads to the phosphorylation of the bifunctional enzyme (hence lower level of fructose 2,6-bisphosphate), which inhibits the kinase and stimulates the phosphate (slows glycolysis).

When blood glucose is low, the hormone ____________ is secreted.

glucagon

The glucagon signaling pathway leads to the phosphorylation of what?

the bifunctional enzyme, which inhibits the kinase and stimulates the phosphatase

Describe this diagram.

This shows control of the synthesis and degradation of fructose 2,6-bisphosphate.

The formation and breakdown of fructose 2,6-bisphosphate is catalyzed by the same protein dependant upon whether it is phosphorylated or not.

Explain how fructose-2,6-bisphosphate (F2,6BP) takes allosteric control of carbohydrate metabolism.

• allosteric activator of phosphofructokinase (PFK)

• allosteric inhibitor of fructose bisphosphate phosphatase (FBPase)

What does high concentration of F2,6P stimulate? How about low concentration?

High concentration of F2,6P stimulates glycolysis; a low concentration stimulates gluconeogenesis

What does concentration of F2,6P in a cell depend on?

the balance between its synthesis (catalyzed by phosphofructokinase-2) and its breakdown (catalyzed by fructose bisphosphatase-2)

What is each enzyme in carbohydrate metabolism controlled by?

phosphorylation/dephosphorylation

Why does metabolism involve cycling of glucose?

due to glycolysis in muscle and gluconeogenesis in liver

What does glycolysis in fast-twitch skeletal muscle produce?

lactate in conditions of O2 debt

After what event does gluconeogenesis recycle lactate from glycolysis?

after it is first oxidized to pyruvate (occurs in liver)

How is glucose transported back to skeletal muscle?

by blood (liver shares stress of vigorous exercise)

Describe this diagram.

This shows the Cori Cycle

What would be the effect on an organism’s ability to use glucose as an energy source if a mutation inactivated glucose 6-phosphatase in the liver?

An important metabolic role of the liver is the release of glucose into the blood for use by other tissues such as muscle, brain, and red blood cells. Now that the role of the liver is established, we can ask what part glucose 6-phosphatase plays in this role. Since glucose 6-phosphatase is an enzyme, the way to get at its role is to find out what it does.

The reaction catalyzed by glucose 6-phosphatase is

• Glucose 6-phosphate + H2O → glucose + Pi

The enzyme catalyzes the last step in gluconeogenesis, and the free glucose is released into the blood.

The blood glucose concentration would fall, resulting in energy deprivation for glucose-dependent tissues.

Regulation of glycolysis and gluconeogenesis is complex. It involves allosteric interactions, covalent modifications, and control of enzymes that make/break down molecules that affect the pathway.

Name the enzymes regulated in each pathway.

Glycolysis- hexokinase, PFK 1, pyruvate kinase

Gluconeogenesis- pyruvate carboxylase, PFPCK, F1,6BPase, G6Pase

Regulation of glycolysis and gluconeogenesis is complex. It involves allosteric interactions, covalent modifications, and control of enzymes that make/break down molecules that affect the pathway.

Name allosteric activators and allosteric inhibitors of the reciprocally regulated enzymes.

Please note that at high energy level, the need for the citric acid cycle is less. Under this condition, citrate accumulates and signal that energy is high which inhibit glycolysis and activate Fructose - 1,6,bisphosphatase in gluconeogenesis. Acetyl CoA also activates pyruvate carboxylase in gluconeogenesis

Regulation of glycolysis and gluconeogenesis is complex. It involves allosteric interactions, covalent modifications, and control of enzymes that make/break down molecules that affect the pathway.

Name the most important allosteric regulator of the two pathways and show precisely how its synthesis and breakdown is regulated.

Most important regulator is Fructose-2,6-bisphosphate (F2,6BP). F2,6BP is synthesized and degraded by different active sties on the bifunctional enzyme PFK-2/FBPase-2.

• Its synthesis is activated by dephosphorylation of FBPase 2/PFK2 by phosphoprotein phosphatase (ultimately activated by insulin)

• Its breakdown is activated by phosphorylation of FBPase2/PFK2 by protein kinase (ultimately activated by glucagon)

High concentration of F2,6P stimulates glycolysis; a low concentration stimulates gluconeogenesis.

Which of the following statements about gluconeogenesis are true?

a) It occurs actively in the muscle during periods of exercise.

b) It occurs actively in the liver during periods of exercise or fasting

c) It occurs actively in adipose tissue during feeding

d) It occurs actively in the kidney during periods of fasting.

b) It occurs actively in the liver during periods of exercise or fasting

d) It occurs actively in the kidney during periods of fasting.

Match the capital letters indicating the reactions of the gluconeogenic pathway with the following statements.

In the coordinated control of phosphofructokinase (PFK) and fructose 1,6-bisphosphatase (F-1,6- BPase),

a) citrate inhibits PFK and stimulates F-1,6-BPase

b) fructose 2,6-bisphosphate inhibits PFK and stimulates F-1,6-BPase.

c) AMP inhibits PFK and simulates F-1,6-BPase

d) Acetyl- CoA inhibits PFK and stimulates F-1,6-BPase

a) citrate inhibits PFK and stimulates F-1,6-BPase

Which of the following statements about the Cori cycle and its physiological consequences are true?

a) It involves the synthesis of glucose in muscle.

b) It involves the release of lactate by muscle

c) It involves lactate synthesis in the liver

d) It involves ATP synthesis in muscle

e) It involves the release of glucose by the liver

b) It involves the release of lactate by muscle

d) It involves ATP synthesis in muscle

e) It involves the release of glucose by the liver

Glucose can be synthesized from which of the following noncarbohydrate precursors?

a) adenine

b) alanine

c) lactate

d) palmitic acid

e) glycerol

b) alanine

c) lactate

e) glycerol

How many “high-energy” bonds are required to convert oxaloacetate to glucose?

a) 2

b) 3

c) 4

d) 5

e) 6

c) 4