HBC 331Review and Integration of

Metabolism

Nisson Schechter PhDDepartment of Biochemistry and Cell

BiologyDepartment of PsychiatryHSC: T10, Room 050/049

Telephone# 444-1368FAX# 444-7534

nisson.schechter@stonybrook.edu

Review and Integration of Metabolism

• Key lecture objectives:– Recognize “the big picture” of

metabolism.

Don’t panic but, you need to know the details to get the “big Picture”.

Review and Integration of Metabolism

• Key lecture objectives:– Recall and recount the fate of glucose

in the presence and absence of O2.– Recall and recount how PFK is

regulated.– Compare and contrast glycolysis vs.

gluconeogenesis.– Outline the flow of molecules to and

from pyruvate and acetyl-CoA.

Review and Integration of Metabolism

• Key lecture objectives:– Distinguish between the function of

triacylglycerol lipase and acetyl-CoA carboxylase.

– Know the different fuel requirements of brain, muscle, liver, kidney, adipose tissue.

– Know the effect of insulin and glucagon on muscle, liver and adipose tissue under fed, fast, starvation, and exercise conditions.

Review and Integration of Metabolism

• Key lecture objectives:– Distinguish between glycogen

phosphorylase vs. glycogen synthase.– Recount the role of glutamine in the

kidney under starvation conditions.

Metabolic adaptation to starvation, fasting, feeding, fed, and exercise.

What are the metabolic consequences of these states?

Eisenhour’s highway act.

The Assent of Man?

Metabolic Syndrome• Abdominal obesity (excessive fat tissue in and

around the abdomen). • Atherogenic dyslipidemia (blood fat disorders

— high triglycerides, low HDL cholesterol and high LDL cholesterol).

• Elevated blood pressure Insulin resistance or glucose intolerance.

Chronic Inflammation

Pyruvate and acetyl-CoA are central to the major metabolic pathways.

Alanine

Note!

Most of the major metabolic pathways involving energy and fuel utilization can be carried out in the liver.

Other tissues are more metabolically specialized i.e. brain, kidney and muscle.

The metabolic interrelationships among brain, adipose tissue, muscle, liver, and kidney.

Red arrows indicate pathways in the fed state:

The liver is the organ of biochemical integration.

During heavy exercise gluconeogenesis increases. Is this a paradox?

Energy Stores in Man• Chronic high blood glucose levels are

toxic. Why?• Total amount of glucose in blood and liver

is small and is exhausted in minutes. • Mobilization of glycogen, fats, and protein

provide energy in the longer term. • Integration of metabolism prevents

metabolic catastrophes.– Hormonal– Allosteric

Interconversion of macronutrients as a function of starvation

Glycolysis: Metabolic degradation of glucose

• What is achieved (aerobically)?– 2 molecules each of pyruvate, NADH

(oxidized in mitochondria), and ATP.• What is achieved (anaerobically)?

– 2 molecules of pyruvate are converted to lactate and the 2 molecules of NADH are regenerated to NAD+. 2 molecules of ATP are also produced.

Glycolysis

• Glycolysis is largely controlled by Phosphofructokinase-1 (PFK-1).– PFK-1 is activated by AMP and ADP,

whose concentrations rise as the need for cellular energy increases.

– PFK-1 is inhibited by ATP and citrate, whose concentrations rise as the demand for cellular energy decreases.

Glycolysis

• Glycolysis is also regulated by the allosteric effector fructose-2,6-bisphosphate (F2,6P) via the phosphofructokinase (PFK-1).– PFK-1 is activated by fructose-2,6-

bisphosphate.– Fructose-2,6-bisphosphate is regulated by

glucagon, epinephrine, and, norepinephrine via a cAMP cascade.

Glycolysis• Liver and heart muscle F2,6P are

regulated oppositely.– In liver an increase [cAMP] causes a

decrease [F2,6P] so glycolysis is inhibited.

– In heart, an increase in cAMP causes an increase in [F2,6P] so glycolysis is stimulated.

– Skeletal muscle [F2,6P] does not respond to changes in [cAMP].

Feeding stimulates insulin secretion.

Insulin stimulates synthetic (anabolic) reactions.Insulin also stimulates protein synthesis.

Muscle does not have receptors for glucagon.

Fasting stimulates glucagon secretion.

Gluconeogenesis:Metabolic synthesis of glucose.

• Glucose can be synthesized from pyruvate, lactate, glycerol, and glucogenic amino acids mainly in liver and kidney.

• Glucose can not by synthesized from fatty acids - “ya can’t take acetate to glucose”.

• The irreversible steps of glycolysis are are bypassed by 4 gluconeogenic enzymes.

During Starvation–Remember!

• Fatty acids are mobilized to acetyl-CoA.– But, acetate is not a precursor to

glucose. Why?• In the mobilization of FAs glycerol is

produced and it is glucogenic via dihydroxy acetone phosphate.

• Also proteins are degraded and some of them are glucogenic, i.e. alanine.

Glycolysis Gluconeogenesis

Hexokinase Glucose-6-phosphatase

Phosphofructokinase-1 (PFK-1)

Fructose-1,6-bisphosphatase

Pyruvate kinase Pyruvate carboxylase (ATP)

*Phosphoenolpyruvate carboxykinase (GTP)

* Committed reaction of gluconeogenesis

Enzymes of glycolysis vs. gluconeogenesis.

What is the role of malate DH?

Pyruvate is carboxylated in the mitochondria. Pyruvate Carboxylase

Oxaloacetate can’t pass out of the mitochondria.

malate DH

malate DH

Oxaloacetate decarboxylated and phosphorylated in the cytosol.

Phosphoenolpyruvate CarboxykinasePEP

Gluconeogenesis-Additional Points

Fructose-1,6-bisphosphatase and Phosphofructokinase can be simultaneously active so the rate and direction of metabolites through glycolysis and gluconeogenesis are reciprocally regulated.

Glycogen Breakdown and Synthesis

•Occurs in liver and muscle.

•Glycogen phosphorylase VS. glycogen synthase; reciprocally regulated by phosphorylation/dephosphorylation via a cAMP cascade.

Glycogen Breakdown and Synthesis

glycogen phosphorylase glycogen synthase

enzyme phosphorylated = active

enzyme phosphorylated = inactive

*glucagon/epinephrine stimulates

*insulin stimulates

*The glucagon/insulin ratio is a crucial factor in determiningthe rate and direction of glycogen metabolism.

More on Insulin (Fed State)

• In liver-– Insulin decreases the production of

glucose by inhibiting gluconeogenesis and the breakdown of glycogen.•This makes sense. Why make more

glucose when there is plenty around?

• In muscle and liver-– Insulin increases glycogen synthesis.

•This makes sense. Glucose is abundant, so store it.

Fatty Acid (FA) Degradation and Synthesis

• Degradation and synthesis of FA is via acetyl-CoA but by 2 distinct pathways.

-oxidation of FAs is regulated by the hormone sensitive triacylglycerol lipase (phosphorylation/dephosphorylation) via cAMP cascade.

• FA synthesis is regulated by acetyl-CoA carboxylase.

FA Degradation and Synthesis.

triacylglycerol lipase acetyl-CoA carboxylase

enzyme phosphorylated = active

enzyme phosphorylated = inactive

*glucagon/epinephrine stimulates

* insulin stimulates

*Here again, glucagon/insulin ratio is a crucial factor in determining the rate and direction of fatty acid metabolism.

Metabolism of fatty acids differs from tissue to tissue.

• The liver can both synthesize and oxidize (degrade) fatty acids.

• Skeletal muscle can not synthesize fatty acids but is active in fatty acid oxidation.– Acetyl-CoA carboxylase is active

in muscle but its product malonyl CoA is an allosteric regulator.

LIVER

CPT=Carnitine palmitoyltransferase

If [malonyl-CoA] is high FA syn. is ongoing and it inhibits the transfer of FA into the mitochondria so degradation is suppressed.

In muscle malonyl CoA acts as an allosteric regulator. When Energy levels are low the carboxylase is phosphorylated reducing malonyl CoA production. Thus beta oxidation proceeds.

AMP UP

Glucose 6-phosphate has 3 fates.

The liver is the gluconeogenic organ.

Glucose Transporters

The GLUT family of membrane proteins transport glucose into the cell.

Insulin stimulates GLUT4 by movement of vesicles containing the GLUT4-glucose.

Fed State

Organ Specialization

1. Brain2. Muscle3. Adipose tissue4. Liver5. kidney

1. Brain• Brain is about 2% of the adult body mass but

represents about 20% of the O2 consumption at rest.

• Normally, glucose serves as the brain’s only fuel.

• Under extended fasting the brain gradually switches to ketone bodies.

• Brain dysfunction results when blood glucose levels fall below 5mM (like about 1/2). Severe insulin overdose results in coma and death.

• One main function of the liver is to maintain blood glucose levels at 5mM.

2. Muscle

• Muscle’s fuels are glucose (from glycogen or blood stream), FAs, and ketone bodies.

• Rested well fed muscle synthesizes glycogen (a readily available fuel depot).

• Muscle cannot export glucose because is lacks glucose-6-phosphatase.

• However, muscle can serve as an energy reservoir because in the fasting state muscle proteins are degraded to AAs.

2. Muscle

• Many of the AA are converted to pyruvate which is converted to alanine by transamination.

• Alanine is transported to the liver, which transaminates it back to pyruvate, a glucose precursor- the glucose-alanine cycle.

• Part of the metabolic load of the muscle is transferred to the liver.

• Remember- Muscle does not do gluconeogenesis.

2. Muscle

• Muscle does not have receptors for glucagon.

• Muscle does have receptors for epinephrine which simulates glycogen breakdown by phosphorylating glycogen phosphorylase via cAMP cascade.

• Heart muscle is largely aerobic; loaded with mitochondria.

• Heart metabolizes FAs (fuel of choice in resting state), ketone bodies, glucose (heavy exercising), pyruvate, lactate.

3. Adipose Tissue

• Adipose tissue is 2nd in importance to the liver in maintaining metabolic homeostasis.

• Adipose tissue obtains FAs from the liver and diet.

• Adipocytes hydrolyze triacylglycerols (triacylglycerol lipase) to FAs and glycerol depending on glucagon, epinephrine, and insulin.

3. Adipose Tissue

• If glycerol-3-phosphate is abundant the FAs are reesterified to triacylglycerols.

• If If glycerol-3-phosphate is in short supply the FAs are released into the blood stream.

• Note: Adipose cells lack the kinase that phosphorylates glycerol. Glycerol-3-phosphate arises from the reduction of dihydroxyacetone phosphate which is generated from glycolysis. Pyruvate and oxaloacetate can also be a source of glycerol-3-phosphate (glycerolneogenesis).

4. Liver

• Acts as a blood glucose “buffer.”• After a CHO meal and blood

glucose>6mM, the liver takes up glucose and converts it to G6P (glucokinase).

• Liver cells are permeable to glucose. Insulin does not directly effect glucose uptake in liver. It does stimulate the synthesis and availability of glucokinase. In contrast, insulin stimulates glucose uptake in muscle and adipose cells.

4. Liver

• The glucose transporter in liver hepatocytes (GLUT 2) is so effective that the glucose concentration in the hepatocyte is the same as it is in blood.

LIVER

MUSCLE

In the fed state blood glucose rise above 5 mmolar then glucokinase converts glucose to glucose 6-phosphate.

4. Liver

Note: The liver does have receptors for insulin.

• In the fed state:– Insulin stimulates glucokinase

transcription and availability.– Insulin stimulates glycogen synthesis

in liver.– Insulin also inhibits

gluconeogenesis.

*Insulin stimulates glucokinase transcription in liver

**

Cori Cycle

Alanine Cycle

5. Kidney

• Function:– To concentrate and filter out waste

nitrogen in the form of urea– To recover important metabolites

such as glucose.– To maintain the blood’s pH and

electrolyte balance.• Fuel:

– Under normal conditions – lactate> glucose> fatty acids>glutamine.

5. Kidney

– Under conditions of acidosis the major fuel source is glutamine.

Glutamine→ Glutamate → α-ketoglutarate +2NH4+.

The production of 2NH4+ neutralizes

acidemia.The two enzymes are glutaminase and

glutamate DH (oxidative deamination).The α-ketoglutarate is glucogenic and thus is

an energy source for the kidney.

Note on : Alanine and Glutamine

In the blood these two amino acids are in much higher concentration than the other amino acids.

WHY?

5. Kidney

Note:

Under conditions of starvation the α-ketoglutarate enters the gluconeogenic pathway and generates as much as 50% of the body’s glucose supply.

HOW?HINT: α-ketoglutarate DH

So glutamate enters the TCA cycle via α-ketoglutarate and is a significant energy source in kidney.

ACL= ATP citrate lyase

The influence of XS ethanol on metabolism.

GLUCOSE

ALANINE PALMITATE