Gluconeogenesis - Biochemistry - Lecture Slides, Slides of Biochemistry

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Gluconeogenesis

Reading:

 Harper’s Biochemistry Chapter 21

 Lehninger Principles of Biochemistry

3rd Ed. pp. 723-

OBJECTIVES

1. To understand how blood glucose levels are

regulated by hormones, especially epinephrine,

glucagon, and insulin.

2. To examine metabolic consequences of loss of

glucose homeostasis.

3. To understand how glucose is synthesized from

other substrates, and which substrates can be

used for this purpose.

4. To understand how glycolysis and

gluconeogenesis are coordinately regulated so as

to avoid futile cycles in the cell.

Biomedical Importance

 The biosynthesis of glucose is an absolute necessity of all mammals, because the brain and nervous system, as well as erythrocytes, testes, renal medulla, and embryonic tissue, require glucose from the blood as their sole or major fuel source.

 The human brain alone requires 120 g of glucose each day. Below a critical blood glucose concentration (normal = 65- 110 mg/dL or 3.6-6 mM), brain dysfunction can occur which can lead to coma and death.

 Even when fat may be supplying most of the caloric requirements of an organism, there is always a certain basal requirement for glucose e.g. in skeletal muscle under anaerobic conditions.

 Glucose is precursor of lactose in the mammary gland.

 Gluconeogenic mechanisms are used to clear lactate (from muscle and erythrocytes) and glycerol (adipose tissue) from blood. Docsity.com

Gluconeogenesis vs. Glycolysis

 Thermodynamic barriers prevent a simple reversal of glycolysis in conversion of pyruvate to glucose.

 7 of 10 reactions of gluconeogenesis are the reverse of glycolytic reactions.

 Three reactions of glycolysis are essentially irreversible in vivo and cannot be used in gluconeogenesis

• the conversion of glucose to glucose 6-phosphate
• the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate by phosphofructokinase-
• the conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase.

 In cells, these three reactions are characterized by a large negative G, whereas other glycolytic reactions have a G near zero and can be reversed in vivo during gluconeogenesis. Docsity.com

BYPASS REACTIONS

1. Conversion of pyruvate to

phosphoenolpyruvate

 Pyruvate is first transported into the mitochondria from the cytosol, or generated from alanine by transamination within mitochondria. Pyruvate is converted to oxaloacetate by pyruvate carboxylase which requires biotin: Pyruvate + HCO 3 -^ + ATPoxaloacetate + ADP + Pi

 Pyruvate carboxylase requires acetyl-CoA as a positive effector, and biotin acts as a carrier of activated HCO 3 -

 The oxaloacetate formed is reduced to malate by mitochondrial malate dehydrogenase: oxaloacetate + NADH + H+^ L-malate + NAD+

 Malate leaves the mitochondrion and is re-oxidized to oxaloacetate, with production of cytosolic NADH Malate + NAD+^ oxaloacetate + NADH + H+ Docsity.com

 This is catalyzed by Mg2+^ -dependent fructose

1,6-bisphosphate which hydrolyzes the C-

phosphate

fructose 1,6-bisphosphate + H 2 O

fructose 6-phosphate

G´° = -16.3 kJ/mol

BYPASS REACTIONS

2. Conversion of fructose 1,6-bisphosphate

to fructose 6-phosphate

 The biosynthetic pathway of gluconeogenesis allows

the net synthesis of glucose from pyruvate and also

from the citric acid cycle intermediates citrate,

isocitrate, -ketoglutarate, succinyl-CoA, succinate,

fumarate, and malate, all of which can undergo

oxidation to oxaloacetate.

 Some or all of the carbon atoms of many of the amino

acids derived from protein are ultimately converted to

pyruvate or to intermediates in the citric acid cycle.

Such amino acids are said to be glucogenic.

 Alanine and glutamine are particularly important

because they are the primary molecules that transport

amino groups from extrahepatic tissue to the liver.

Citric acid cycle intermediates and

many amino acids are glucogenic

Reciprocal regulation of

gluconeogenesis and glycolysis

 1st control point - fate of pyruvate

 Two alternative fates for pyruvate. Pyruvate can be converted to glucose and glycogen via gluconeogenesis or oxidized to acetyl-CoA for energy production. The first enzyme in each path is regulated allosterically; acetyl-CoA stimulates pyruvate carboxylase and inhibits the pyruvate dehydrogenase complex Docsity.com

 2nd control point - fructose 1,6-bisphosphate and phosphofructokinase

Glucose

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Citric Acid Cycle

ATP

citrate ATP AMP, ADP

AMP

 Fructose 2,6-bisphosphate activates PFK-1 and

inhibits FBPase-1, stimulating glycolysis and

inhibiting gluconeogenesis

 Fructose 2,6-bisphosphate levels are regulated by rates of synthesis by PFK-2 and breakdown by FBPase-

 Regulation of fructose 2,6-bisphosphate level, (a) The cellular concentration of the regulator fructose 2,6- bisphosphate is determined by the rates of its synthesis by PFK-2 and breakdown by FBPase-2. (b) Both of these enzymes are part of the same polypeptide chain, and both are regulated, in a reciprocal fashion, by glucagon. Here and elsewhere, arrows are used to indicate increasing and decreasing levels of metabolites.