Glycolysis, Gluconeogenesis, and Glycogen Metabolism: Key Pathways in Carbohydrate Biochemistry

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Glycolysis: An Overview

Phases of Glycolysis

Glycolysis is a central metabolic pathway that converts glucose into pyruvate, generating energy in the form of ATP and NADH. The pathway consists of 10 enzyme-catalyzed reactions, which are grouped into two distinct phases:

Energy-Investment Phase: Two ATP molecules are consumed to phosphorylate glucose and convert it into two triose phosphates.
Energy-Generation Phase: The triose phosphates are oxidized to two pyruvate molecules, producing four ATP and two NADH.

Net Reaction of Glycolysis:

Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP + 2 H2O + 2 H+

Reactions of Glycolysis

Stepwise Enzymatic Reactions

Hexokinase: Phosphorylates glucose to glucose-6-phosphate using ATP. ΔG°' = -18.4 kJ/mol
Glucose 6-Phosphate Isomerase: Converts glucose-6-phosphate to fructose-6-phosphate. ΔG°' = +1.7 kJ/mol
Phosphofructokinase (PFK): Phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate. This is a major control point and an allosteric enzyme. ΔG°' = -15.9 kJ/mol
Aldolase: Cleaves fructose-1,6-bisphosphate into two triose phosphates (dihydroxyacetone phosphate and glyceraldehyde-3-phosphate). ΔG°' is positive, but under cellular conditions, the reaction proceeds as written.
Triose Phosphate Isomerase: Interconverts dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. ΔG°' = +7.6 kJ/mol
Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH): Oxidizes glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, producing NADH. ΔG°' = +6.3 kJ/mol
Phosphoglycerate Kinase: Transfers a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP (substrate-level phosphorylation). ΔG°' = -17.2 kJ/mol
Phosphoglycerate Mutase: Converts 3-phosphoglycerate to 2-phosphoglycerate. ΔG°' = +4.4 kJ/mol
Enolase: Dehydrates 2-phosphoglycerate to phosphoenolpyruvate (PEP) via α,β-elimination. ΔG°' = -3.2 kJ/mol
Pyruvate Kinase: Transfers a phosphate from PEP to ADP, forming ATP and pyruvate (second substrate-level phosphorylation). ΔG°' = -29.7 kJ/mol

Mechanistic Note

The synthesis of ATP by pyruvate kinase is overall exergonic due to the spontaneous tautomerization of enolpyruvate to the more stable keto form (pyruvate).

Anaerobic Fates of Pyruvate

Regeneration of NAD+

For glycolysis to continue, NADH must be reoxidized to NAD+.
Under anaerobic conditions, pyruvate is reduced to lactate (in animals) or ethanol (in yeast), regenerating NAD+.

Examples:

Lactate fermentation: Animal cells, lactic acid bacteria
Alcoholic fermentation: Yeast

Gluconeogenesis

Glucose Synthesis and Use

The human brain requires ~120 g/day of glucose; total body requirement is ~160 g/day.
Glycogen reserves and glucose in body fluids provide about one day's supply.
When glucose is depleted, it must be synthesized from non-carbohydrate precursors via gluconeogenesis.

Pathway Overview

Gluconeogenesis is essentially glycolysis in reverse, but three irreversible glycolytic reactions must be bypassed:

Irreversible Glycolysis Enzyme	Gluconeogenesis Bypass Enzyme
Hexokinase	Glucose-6-phosphatase
Phosphofructokinase	Fructose-1,6-bisphosphatase
Pyruvate kinase	Pyruvate carboxylase & phosphoenolpyruvate carboxykinase

Cori Cycle

The liver is the most active gluconeogenic organ.
The Cori cycle describes the recycling of lactate produced by anaerobic glycolysis in muscle back to glucose in the liver.

Coordinated Regulation of Glycolysis and Gluconeogenesis

Reciprocal Control

Glycolysis and gluconeogenesis are reciprocally regulated to prevent futile cycles.
Regulation also maintains pools of intermediates for biosynthetic purposes.

Major Control Points

Allosteric activators and inhibitors regulate key exergonic reactions in both pathways.
Examples of control points include:
- Glucose-6-phosphate
- Fructose-2,6-bisphosphate
- AMP, ADP, ATP, citrate
- Acetyl-CoA
- Hormonal regulation by insulin and glucagon

Glycosidic Bond Cleavage of Disaccharides

Hydrolysis and Phosphorolysis

Dietary polysaccharides are metabolized by hydrolysis to monosaccharides.
Intracellular carbohydrate stores (e.g., glycogen) are mobilized as phosphorylated monosaccharides by phosphorolysis.

Digestion of Amylopectin or Glycogen

Enzymatic Cleavage

α-amylase in saliva cleaves α(1→4) linkages from the nonreducing ends but cannot cleave α(1→6) linkages at branch points.
For complete digestion, α(1→6)-glucosidase (a debranching enzyme) removes the limit dextrin, exposing additional linkages.

Glycogen Metabolism in Muscle and Liver

Glycogen Utilization

Glycogen phosphorylase cleaves α(1→4) bonds via phosphorolysis, yielding α-D-glucose-1-phosphate.
α-D-glucose-1-phosphate is converted into α-D-glucose-6-phosphate by phosphoglucomutase for entry into glycolysis or other pathways.

Debranching Process

Glucantransferase transfers three glucose residues from the limit branch to another nonreducing end.
α(1→6)-glucosidase removes the remaining glucose molecule at the branch point.

Glycogen Synthesis

UDP-glucose is an activated form of glucose for glycogen synthesis.
Glycogen synthase uses UDP-glucose to synthesize α(1→4)-linked glycogen.
Branching enzyme (amylo-(1,4→1,6)-transglycosylase) creates branches by transferring residues to form α(1→6) linkages.

Summary Table: Key Enzymes and Functions

Enzyme	Function
Hexokinase	Phosphorylates glucose
Phosphofructokinase	Major control point, phosphorylates F6P
Glycogen phosphorylase	Cleaves glycogen via phosphorolysis
Glycogen synthase	Synthesizes glycogen from UDP-glucose
Branching enzyme	Creates α(1→6) branches in glycogen

Additional info: These notes expand on the original slides by providing definitions, equations, and context for each step and regulatory mechanism, suitable for exam preparation in a college biochemistry course.