14 (part II)| Gluconeogenesis

© 2017 W. H. Freeman and Company

Carbohydrates other than glucose also feed into glycolysis

• Glucose molecules are cleaved from glycogen and starch by glycogen phosphorylase.• yielding glucose-1-phosphate

• uses inorganic phosphate as phosphate source

• Disaccharides are hydrolyzed.• lactose: glucose and galactose

• sucrose: glucose and fructose

• Monosaccharides fructose, galactose, and mannose enter glycolysis at different points.

Entry of dietary glycogen, starch, disaccharides, and hexoses into the preparatory stage of glycolysis.

Lactose intolerance occurs because most adults lack the enzyme to degrade lactose.

Milk (lactose) intolerance

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Fates of Pyruvate

Gluconeogenesis is the synthesis of glucose from noncarbohydrate precursors. Most common precursors in human body are pyruvate, lactate, glycerol, and amino acids.

The major site of gluconeogenesis is the liver, although gluconeogenesis can occur in the kidney.

Gluconeogenesis is especially important during fasting or starvation, as glucose is the primary fuel for the brain and the only fuel for red blood cells.

Gluconeogenesis: Making “New” Glucose

Gluconeogenesis converts pyruvate into glucose.

Gluconeogenesis occurs mainly in the liver.

Glycolysis occurs mainly in the muscle and brain.

The Cori cycle

Muscle and liver display inter-organ cooperation in a series of reactions called the Cori cycle.Lactate produced by muscle during contraction is released into the blood.Liver removes the lactate and converts it into glucose, which can be released into the blood.

The carbon skeletons of some amino acids can be converted into gluconeogenic intermediates.

Glycerol, derived from the hydrolysis of triacylglycerols, can be converted into dihydroxyacetone phosphate, which can be processed by gluconeogenesis or glycolysis.

Precursors for Gluconeogenesis

• Animals can produce glucose from sugars or proteins.– sugars: pyruvate, lactate, or oxaloacetate– protein: from amino acids that can be converted to citric

acid cycle intermediates (or glucogenic amino acids)

• Animals cannot produce glucose from fatty acids.– product of fatty acid degradation is acetyl-CoA– cannot have a net converstion of acetyl-CoA to

oxaloacetate• Plants, yeast, and many bacteria can do this, thus

producing glucose from fatty acids.

Why Gluconeogenesis is Important?

Human daily glucose requirement: 160 grams (75% is used by brain)

Human glucose reserves: 20 grams in body fluid + 190 grams in the form of glycogen

The three irreversible steps in glycolysis must be bypassed in gluconeogenesis.

Enzymes unique to gluconeogenesis are Pyruvate Carboxylase; PEP Carboxykinase (PEPCK), Fructose 1,6 Bisphosphatase (F1,6BPase), and Glucose 6-phosphatase (G6Pase)

Steps 1 and 2 : Pyruvate to Phosphoenolpyruvate

Requires two energy-consuming steps

Steps 1 and 2 : Pyruvate to Phosphoenolpyruvate

• Requires two energy-consuming steps

• The first step, pyruvate carboxylase converts pyruvate to oxaloacetate.– carboxylation using a biotin cofactor– occurs in the mitochondria

• The second step, phosphoenolpyruvate carboxykinase converts oxaloacetate to PEP.– phosphorylation from GTP and decarboxylation

During this conversion, the same carbon is added and immediately removed from the structure.

Synthesis of Oxaloacetate

• In mitochondria,

pyruvate is converted

to oxaloacetate in a

biotin-requiring

reaction catalyzed by

pyruvate carboxylase.

• Biotin is a CO2

carrier; it is a cofactor

covalently attached to

the enzyme

Oxaloacetate to Phosphoenolpyruvate

• In the cytosol, oxaloacetate

is converted to

phosphoenolpyruvate (PEP)

by PEP carboxykinase.

• The CO2 incorporated in the

pyruvate carboxylase

reaction is lost here as CO2.

The formation of oxaloacetate by pyruvate carboxylase occurs in the mitochondria.

Oxaloacetate is reduced to malate and transported into the cytoplasm, where it is reoxidized to oxaloacetate with the generation of cytoplasmic NADH.

PEP is then synthesized from oxaloacetate by phosphoenolpyruvate carboxykinase.

Additional Bypasses

• Catalyze reverse reaction of opposing step in glycolysis• Are irreversible themselves

• Fructose 1,6-bisphosphate fructose 6-phosphate– by fructose bisphosphatase-1– coordinately/oppositely regulated with PFK– cleaves phosphate with water– DOES NOT generate ATP

• Glucose 6-phosphate glucose– by glucose 6-phosphatase– segregated in the endoplasmic reticulum– DOES NOT generate ATP

Gluconeogenesis Is Expensive

• Costs 4 ATP, 2 GTP, and 2 NADH

• Physiologically necessary: Brain, nervous system, and red blood cells generate ATP ONLY from glucose.

• Allows generation of glucose when glycogen stores are depleted:– during starvation– during vigorous exercise– can generate glucose from amino acids, but not fatty acids

2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 4 H2O Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+

The reciprocal regulation of gluconeogenesis and glycolysis in the liver

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

Additionally, glycolysis and gluconeogenesis are reciprocally regulated at the interconversion of phosphoenolpyruvate and pyruvate.

If ATP is required, glycolysis predominates. If glucose is required, gluconeogenesis is favored.

In liver, the rates of glycolysis and gluconeogenesis are adjusted to maintain blood glucose levels.

The key regulator of glucose metabolism in liver is fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate stimulates phosphofructokinase and inhibits fructose 1,6-bisphosphatase.

The kinase that synthesizes fructose 2,6-bisphosphate (phosphofuctokinase 2) and the phosphatase that hydrolyzes this molecule (fructose bisphosphatase) are located on the same polypeptide chain. Such an arrangement is called a bifunctional enzyme.

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A. 1,2,3

B. 1,2,4

C. 1,4,5

D. 1,3,4

E. 2,3,4

1. Hexokinase

2. Phosphoglycerate kinase

3. Phosphofructokinase 1

4. Pyruvate kinase

5. Triosephosphate isomerase

There are three irreversible steps in glycolysis that must be bypassed in gluconeogenesis. What are three enzymes in glycolysis catalyzing that

irreversible steps?

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A. 1,2,3

B. 1,2,4

C. 1,4,5

D. 1,3,4

E. 2,3,4

1. Hexokinase

2. Phosphoglycerate kinase

3. Phosphofructokinase 1

4. Pyruvate kinase

5. Triosephosphate isomerase

There are three irreversible steps in glycolysis that must be bypassed in gluconeogenesis. What are three enzymes in glycolysis catalyzing these

irreversible steps?

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