Carbohydrate Metabolism

Chapter 8

Metabolism Section 8.1: Glycolysis Section 8.2: Gluconeogenesis Section 8.3: The Pentose Phosphate Pathway Section 8.4: Metabolism of Other Important

Sugars Section 8.5: Glycogen Metabolism

Carbohydrate Metabolism

Overview

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Metabolism and Jet Engines Carbohydrate Metabolism: Biochemical processes

responsible for the formation, breakdown and interconversion of carbohydrates in living organisms

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Energy transforming pathways of carbohydrate metabolism include: glycolysis,

Glycogenesis Glycogenolysis Gluconeogenesis pentose phosphate

pathway

Section 8.1: Glycolysis

Glycolysis occurs in almost every living cell Ancient process central to all life

Splits glucose into two three-carbon pyruvate unitsCatabolic process that captures some energy as 2 ATP and 2 NADH

Figure 8.2 Major Pathways in Carbohydrate Metabolism

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

ATP

Adenosine triphosphate (ATP) is a nucleoside triphosphate used in cells as a coenzyme to store and transport chemical energy

NAD+/NADH

Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells that transfers electrons between reactions in metabolism.

Nicotinamide adenine dinucleotide phosphate

• NADPH is the reduced form of NADP+. • NADP+ differs from NAD+ in the presence of an additional phosphate group on

the 2' position of the ribose ring that carries the adenine moiety.• NADPH is a cofactor used in anabolic reactions, such as lipid and nucleic acid

synthesis, which require NADPH as a reducing agent.

Glycolysis is an anaerobic process

Pyruvate

The conjugate base of pyruvic acid

It is a key intermediate in several metabolic pathways

Made from glucose through glycolysis

Converted back to carbohydrates (such as glucose) via gluconeogenesis, or to fatty acids through acetyl-CoA.

Used to construct the amino acid alanine, and be converted into ethanol or lactic acid via fermentation.

Pyruvic Acid

Alanine

Section 8.1: Glycolysis

The Fates of Pyruvate Under aerobic conditions, pyruvate is converted

to acetyl-CoA for use in the citric acid cycle and electron transport chain

Figure 8.7 The Fates of Pyruvate

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.1: Glycolysis

The Fates of Pyruvate Under anaerobic conditions pyruvate can

undergo fermentation: alcoholic or homolactic

Figure 8.7 The Fates of Pyruvate

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.1: Glycolysis

Energetics of Glycolysis In red blood cells, only three reactions have significantly negative DG values

Many reactions are reversible

Figure 8.9 Free Energy Changes during Glycolysis in Red Blood Cells

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.1: Glycolysis

Regulation of Glycolysis The rate of the glycolytic pathway in a cell is

controlled by the allosteric enzymes: Hexokinases I, II, and III Phosphofructokinase 1 (PFK-1) Pyruvate kinase

Reactions catalyzed by these allosteric enzymes can be turned on or off by activator or inhibitor molecules

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.1: Glycolysis

Regulation of Glycolysis Continued Pyruvate kinase (converts PEP to pyruvate)

activated by high AMP concentrations, inhibited ATP

PFK-1 is activated by fructose-2,6-bisphosphate, produced via hormone- induced covalent modification of PFK-2

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.1: Glycolysis

The peptide hormones glucagon and insulin also regulate glycolysis/gluconeogenesis pathways• Glucagon is released by pancreatic alpha-cells when blood glucose is low;

triggers a series of reactions that inhibit glycolysis and activate gluconeogenesis• Insulin is released by pancreatic beta-cells when blood glucose is high; triggers

a series of reactions that activate glycolysis and inhibit gluconeogenesis

Section 8.2: Gluconeogenesis

Gluconeogenesis is the formation of new glucose molecules from precursors in the liverOccurs when blood sugar levels are low and liver glycogen is depleted

Precursor molecules include lactate, pyruvate, and a-keto acids

Gluconeogenesis Reactions Reverse of glycolysis except the three

irreversible reactions

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

1

2

3 Gluconeogenesis Reactions

Three bypass reactions:1. Synthesis of phosphoenolpyruvate (PEP) via the enzymes pyruvate carboxylase and pyruvate carboxykinase2. Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate via the enzyme fructose-1,6-bisphosphatase3. Formation of glucose from glucose-6-phosphate via the liver and kidney-specific enzyme glucose-6-phosphatase

Section 8.2: Gluconeogenesis

Gluconeogenesis Regulation Rate of gluconeogenesis is affected primarily by: substrate availability allosteric effectors Hormones (e.g., cortisol and insulin)

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.2: Gluconeogenesis

Gluconeogenesis Substrates Three of the most important substrates for gluconeogenesis are:

1. Lactate—released by skeletal muscle following exercise After transfer to the liver lactate is converted to pyruvate, then to glucose

2. Glycerol—a product of fat metabolism. Must be converted to their intermediate glyceraldehyde 3-phosphate to produce glucose.

3. Alanine—generated from pyruvate in exercising muscle Alanine is converted to pyruvate and then glucose in the liver

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Cori Cycle: Lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is metabolized back to lactate.

Allosteric Regulation

allosteric effectors may activate (+) or inhibit (-) steps along the metabolic pathways

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Hormonal Regulation

The peptide hormones glucagon and insulin also regulate glycolysis/gluconeogenesis pathways• Glucagon is released by pancreatic alpha-cells when blood glucose is low;

triggers a series of reactions that inhibit glycolysis and activate gluconeogenesis• Insulin is released by pancreatic beta-cells when blood glucose is high; triggers

a series of reactions that activate glycolysis and inhibit gluconeogenesis

Alternate glucose metabolic pathway Products are NADPH and ribose-5-phosphate Two phases: oxidative and nonoxidative

Pentose Phosphate Pathway

Section 8.3: Pentose Phosphate Pathway Pentose Phosphate Pathway

Alternate glucose metabolic pathway Most active in cells where large quantities of lipids

are synthesized (e.g., adipose tissue, adrenal cortex, mammary glands, liver)

Two phases: oxidative and nonoxidative Oxidative phase:

Produces ribulose-5-phosphate and two NADPH (Nicotinamide adenine dinucleotide phosphate)

NADPH is a reducing agent used in anabolic (synthesis) processes (e.g., to produce lipids)

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Pentose Phosphate Pathway – Oxidative Phase

Figure 8.15a The Pentose Phosphate Pathway (oxidative)

Glucose-6-phosphate dehydrogenase

Gluconolactonase

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Oxidation

Hydrolysis

Figure 8.15a The Pentose Phosphate Pathway (oxidative)

Section 8.3: Pentose Phosphate Pathway

6-phosphogluconate dehydrogenase

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Oxidation

Decarboxylation

Pentose Phosphate Pathway: Nonoxidative Produces important

intermediates for nucleotide biosynthesis and glycolysis

Ribose-5-phosphate Glyceraldehyde-3-

phosphate Fructose-6-phosphate

Figure 8.15b The Pentose Phosphate Pathway (nonoxidative)

Section 8.3: Pentose Phosphate Pathway

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Pentose Phosphate Pathway If the cell requires

more NADPH than ribose molecules, products of the nonoxidative phase can be shuttled into glycolysis

Figure 8.16 Carbohydrate Metabolism: Glycolysis and the Phosphate Pathway

Section 8.3: Pentose Phosphate Pathway

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.4: Metabolism of Other Important Sugars

Fructose, mannose, and galactose also important sugars for vertebrates Most common sugars found in oligosaccharides besides glucose

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

70%

22%

Section 8.4: Metabolism of Other Important Sugars

Fructose Metabolism Second to glucose in the human diet Found in fruit, honey, sucrose, high fructose

corn syrup Can enter the glycolytic pathway in two ways:

Through the liver (multi-enzymatic process) Muscle and adipose tissue (hexokinase)

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.4: Metabolism of Other Important Sugars

Figure 8.17 Carbohydrate Metabolism: Other Important Sugars

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Glycogenesis Synthesis of glycogen, the storage form of glucose, occurs after a

meal Requires a set of three reactions (1 and 2 are preparatory and 3 is

for chain elongation):1. Synthesis of glucose-1-phosphate (G1P) from glucose-6-phosphate by phosphoglucomutase2. Synthesis of uridine diphosphate (UDP)-glucose from G1P by UDP-glucose phosphorylase3. Synthesis of Glycogen from UDP-glucose

Requires two enzymes: Glycogen synthase to grow the chain Amylo-α(1,46)-glucosyl transferase which creates the

α(1,6) linkages for branching

Section 8.5: Glycogen Metabolism

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Glycogenesis ContinuedGlycogen synthase catalyzes addition of glucose onto existing oligosaccharide by transferring glucosyl group of UDP-glucose to non-reducing end of glycogen

Figure 8.18 Glycogen Synthesis

Section 8.5: Glycogen Metabolism

Glycogen synthase

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Branching enzyme

Section 8.5: Glycogen Metabolism

Glycogenesis Continued Branching enzyme

amylo-a(1,41,6)-glucosyl transferase creates a(1,6) linkages for branches that occur ~10 residues apart

Figure 8.18 Glycogen Synthesis

a(1,6) Glycosidic Linkage is formed

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Glycogenolysis Glycogen degradation requires two reactions:

1. Removal of glucose from nonreducing ends (glycogen phosphorylase), which stops when four glucose units remain at the branch point (further degradation to the branch point results in a limit dextrin)

2. 2. Hydrolysis of the a(1,6) glycosidic bonds at branch points by amylo-a(1,6)-glucosidase (debranching enzyme)

Section 8.5: Glycogen Metabolism

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Glycogen phosphorylase uses organic phophate to cleave the a(1,4) linkages on the outer branches

Section 8.5: Glycogen Metabolism

Glycogenolysis Cont.

amylo-a(1,6)-glucosidase first transfers the outer 3 glucose residues to a nearby non-reducing end, then uses water to hydrolyze the a(1,6) glycosidic bonds and remove the last glucose at the branch point

Amylo-a(1,6)-glucosidase

Amylo-a(1,6)-glucosidase

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism

Amylo-a(1,6)-glucosidase

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Glucose-1-phosphate, the major product of glycogenolysis, is diverted to glycolysis in muscle cells to generate energy for muscle contraction.In hepatocytes, G1P is converted to glucose and released into the blood.

Regulation of Glycogen Metabolism Carefully regulated

by synthesis to maintain consistent energy levels

Regulation involves insulin, glucagon, epinephrine, and allosteric effectors

Section 8.5: Glycogen Metabolism

Figure 8.22 Major Factors Affecting Glycogen Metabolism

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Figure 8.22 Major Factors Affecting Glycogen Metabolism

Section 8.5: Glycogen Metabolism

Glucagon activates glycogenolysis

Insulin inhibits glycogenolysis and activates glycogenesis

Epinephrine release activates glycogenolysis and inhibits glycogenesis

From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Regulation of Glycogen Metabolism

Glucagon is released from pancreas when blood glucose levels drop (e.g., after a meal). It binds to hepatocyte receptors, triggering a cascade of reactions that initiate glycogenolysis, releasing glucose into the bloodstream.

Insulin inhibits enzymes of glycogenolysis, and activates glycogenesis enzymes, increasing rate of glucose uptake.

Epinephrine, released due to emotional or physical stress, promotes glycogenolysis and inhibits glycogenesis; provides massive glucose release for flight-or-fight response.