Biology Chapter 8: Glycolysis

Unit II: Intermediary Metabolism

• Glycolysis is a pathway occurring in all tissues
• Glucose is broken down to produce energy in the form of ATP and other intermediates for metabolic pathways
• It's central to carbohydrate metabolism, converting various sugars into glucose
• Pyruvate is the final product of glycolysis in cells with mitochondria (aerobic conditions)
• In anaerobic conditions, pyruvate is converted to lactate

Chapter 8: Glycolysis

• Glycolysis occurs in all cells
• It breaks down glucose to produce ATP and other metabolic intermediates
• The process involves a series of enzymatic reactions
• Aerobic glycolysis occurs in cells with mitochondria with adequate oxygen supply. The end product is pyruvate.
• In anaerobic conditions, pyruvate is converted to lactate
• Glucose transporters (GLUTs) are crucial for glucose uptake
• Various GLUTs exist, each with a different tissue distribution, affinity, and regulation

Transport of Glucose into Cells

• Glucose cannot diffuse into cells; it requires transporters
• Sodium-independent facilitated diffusion transports glucose
• Monomeric proteins (GLUTs) form transport channels across the cell membrane.
• Extracellular glucose binds to the transporter, changing its conformation to allow glucose transport across the cell membrane.
• GLUT-1 is highly concentrated in red blood cells and blood brain barrier, but not in the muscles
• GLUT-2 is present in liver, kidney and pancreatic cells
• GLUT-3 is present in neurons
• GLUT-4 is influenced by insulin, common in muscle and fat tissue
• Sodium-dependent glucose cotransporters (SGLTs) move glucose against a concentration gradient, coupling its transport with sodium ions. This is crucial in the intestine and kidneys.

Reactions of Glycolysis

• Conversion of glucose to pyruvate occurs in two stages.
• Energy investment phase, using ATP to synthesize phosphorylated intermediates
• Energy generation phase, producing a net gain of ATP molecules through substrate-level phosphorylation.
• The initial five reactions form the energy investment stage of glycolysis. The following reactions represent the energy generation phase of glycolysis.
• The conversion of glucose to pyruvate involves a net gain of 2ATP per glucose molecule.

Phosphorylation of Glucose

• Phosphorylated glucose molecules cannot diffuse out of cells due to their negative charge.
• This 'traps' glucose within cells.
• This reaction involves an enzyme called hexokinase.
• Several hexokinase isozymes (I-IV) exist with differing affinities for glucose and varied regulatory mechanisms.
• Reaction controlled by Km and Vmax.

Hexokinase IV (Glucokinase)

• Specifically found in liver and pancreatic cells
• Acts as a glucose sensor for pancreatic beta cells, influencing insulin secretion and responding to hypoglycemia in neurons.
• Participates in glucose metabolism during high blood glucose levels.
• High Km, allowing liver to utilize glucose even at low concentrations within blood and prevents excessive blood glucose levels after a meal

Regulation by Fructose 6-Phosphate and Glucose

• Glucokinase activity is not allosterically inhibited by G-6-P as in other hexokinases.
• Glucokinase controlled by regulatory protein (GKRP), which keeps glucokinase inactive in the nucleus under low glucose conditions.
• High glucose levels cause GK to detach from GKRP, allowing it enter the cytoplasm, where it can phosphorylate glucose to produce glucose-6-phosphate.
• Fructose-1-phosphate inhibits the formation of the GK-GKRP complex

Regulation by Fructose 2,6-Bisphosphate

• Fructose 2,6-bisphosphate is a potent activator of phosphofructokinase-1 (PFK-1) and enhances glycolysis at high ATP levels, playing a critical signaling role.
• Formed from fructose 6-phosphate by phosphofructokinase-2 (PFK-2)
• Dephosphorylation of PFK-2 reverses the process and enhances gluconeogenesis in low glucose states.
• It works in opposition to fructose 1,6-bisphosphatase, ensuring that both glycolysis and gluconeogenesis do not operate simultaneously.

Cleavage of Fructose 1,6-Bisphosphate

• Aldolase A cleaves fructose 1,6-bisphosphate into two 3-carbon molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate,
• Reversible reaction, not a major regulatory step
• In liver and kidney, Aldolase B is involved in metabolizing dietary fructose

Isomerization of Dihydroxyacetone Phosphate

• Triose phosphate isomerase interconverts dihydroxyacetone phosphate into glyceraldehyde-3-phosphate
• Reversible and readily occurs

Oxidation of Glyceraldehyde 3-Phosphate

• Catalyzed by dehydrogenase in Glycolysis which requires NAD+
• Produces the high-energy intermediate 1,3-bisphosphoglycerate via an oxidation reaction coupled to the attachment of inorganic phosphate
• NADH are reoxidized either by the production of lactate or by the respiratory chain

Synthesis of 3-Phosphoglycerate

• Phosphoglycerate kinase catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, producing ATP through substrate-level phosphorylation.
• Important step in the glycolysis, producing energy in the form of ATP

Shift of Phosphate Group & Dehydration

• 3-Phosphoglycerate is rearranged to 2-phosphoglycerate by phosphoglycerate mutase.
• 2-phosphoglycerate is dehydrated creating high-energy intermediate, phosphoenolpyruvate by enolase
• Reversible steps of glycolysis

Formation of Pyruvate

• Conversion of phosphoenolpyruvate to pyruvate is catalyzed by pyruvate kinase, this reaction produces ATP through substrate level phosphorylation
• Pyruvate kinase reaction, irreversible, favored to proceed in the direction towards pyruvate formation in anaerobic conditions

Regulation of Pyruvate Kinase

• Pyruvate kinase activity is regulated by a feedforward mechanism influenced by high levels of fructose-1,6-bisphosphate
• Covalent modulation involves phosphorylation and dephosphorylation of the enzyme. Inactivation occurs via phosphorylation by cAMP-activated protein kinase. Dephosphorylation re-activates the enzyme.
• Covalent modulation is mediated through signaling pathways responsive to glucagon and blood glucose concentrations, crucial for regulating glycolysis and gluconeogenesis, ensuring they do not operate simultaneously.

Genetic Defects of Glycolytic Enzymes

• Defects in genes responsible for glycolysis can result in various symptoms
• Pyruvate kinase deficiency is a common cause of chronic hemolytic anemia (a condition in which red blood cells are destroyed faster than they can be produced)