Define glycolysis. Describe its reactions, energetics and regulation. Add a note on its significance in the red blood cell.

Definition

Reactions

A. Energy investment phase (2 ATP used)

Glucose → Hexokinase / glucokinase — ATP used, irreversible → Glucose-6-phosphate → Phosphohexose isomerase → Fructose-6-phosphate → phosphofructokinase-1 — ATP used, rate-limiting, irreversible → Fructose-1,6-bisphosphate → Aldolase → Glyceraldehyde-3-phosphate + DHAP

• DHAP is converted to G3P by triose phosphate isomerase, so all six carbons continue — hence everything after this point is × 2

B. Energy generation phase (4 ATP made)

Glyceraldehyde-3-phosphate → G3P dehydrogenase — NADH produced; inhibited by iodoacetate → 1,3-bisphosphoglycerate → Phosphoglycerate kinase — ATP made (substrate-level) → 3-phosphoglycerate → Phosphoglycerate mutase → 2-phosphoglycerate → Enolase — inhibited by fluoride → Phosphoenolpyruvate → pyruvate kinase — ATP made, irreversible → pyruvate

The Three Irreversible Steps

• These three are bypassed by separate enzymes in gluconeogenesis

Energetics

• Substrate-level phosphorylation occurs at two steps — phosphoglycerate kinase and pyruvate kinase. These need no oxygen, which is why RBC survive

Regulation

• Fructose-2,6-bisphosphate is the most important physiological activator of PFK-1. It is made by PFK-2, which insulin activates and glucagon inactivates — the link between hormone and pathway
• Citrate inhibits PFK-1 — when the TCA cycle is well supplied, glycolysis slows

Fate of Pyruvate

• The purpose of forming lactate is to regenerate NAD+, without which glycolysis would stop at the G3P dehydrogenase step

Significance

• The only source of ATP for RBC, cornea, lens and renal medulla
• Provides rapid ATP in exercising muscle when oxygen is limiting
• Provides intermediates — DHAP for glycerol-3-phosphate (triglyceride synthesis), 3-phosphoglycerate for serine
• Supplies 2,3-BPG in the RBC through the Rapoport–Luebering shunt
• It is the common pathway for all dietary hexoses — fructose and galactose enter it

Applied Aspects

• Pyruvate kinase deficiency — the second commonest cause of hereditary non-spherocytic haemolytic anaemia; the RBC cannot make enough ATP for its Na+–K+ pump
• Fluoride inhibits enolase — hence sodium fluoride is added to blood collection tubes for glucose estimation, to stop glycolysis in the sample
• Arsenic poisoning — arsenate substitutes for phosphate at the G3P dehydrogenase step, so glycolysis continues but no ATP is formed
• Warburg effect — tumour cells use aerobic glycolysis even with adequate oxygen; the basis of FDG-pet scanning
• Lactic acidosis — from tissue hypoxia, shock, or metformin overdose
• Hexokinase deficiency — a rare haemolytic anaemia; unlike pyruvate kinase deficiency, 2,3-BPG is low, so the anaemia is poorly tolerated
• Cancer chemotherapy increasingly targets glycolytic enzymes, exploiting the tumour cell's dependence on this pathway