Glycolysis

Glycolysis is a ten-step enzymatic pathway located in the cytoplasm. It is divided into the energy investment phase, where 2 ATP are consumed to phosphorylate glucose, and the energy payoff phase, where 4 ATP and 2 NADH are produced. Glucose is initially converted to glucose-6-phosphate by hexokinase or glucokinase, trapping it within the cell. The rate-limiting step is the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by phosphofructokinase-1 (PFK-1), which is allosterically regulated by ATP and citrate. Under aerobic conditions, pyruvate enters the mitochondria to be converted to acetyl-CoA. Under anaerobic conditions, or in cells without mitochondria like erythrocytes, pyruvate is converted to lactate by lactate dehydrogenase, regenerating NAD+ to allow glycolysis to continue. While the net ATP yield is low (2 ATP), the pathway is rapid and can provide energy during high-intensity exercise or cellular hypoxia. Deficiencies in glycolytic enzymes, such as pyruvate kinase, lead to significant clinical syndromes including chronic non-spherocytic haemolytic anaemia.

The glycolytic pathway begins with the phosphorylation of glucose to glucose-6-phosphate (G6P). In most tissues, hexokinase (low Km, high affinity) performs this, while the liver and pancreatic beta cells use glucokinase (high Km, low affinity). This ensures the liver only takes up glucose when blood levels are high. PFK-1 is the primary control point, inhibited by high ATP and citrate levels, but activated by AMP and fructose-2,6-bisphosphate. The latter is regulated by the bifunctional enzyme PFK-2, which responds to the insulin:glucagon ratio. The second half of the pathway involves the cleavage of fructose-1,6-bisphosphate into two 3-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). G3P undergoes oxidation and phosphorylation to produce 1,3-bisphosphoglycerate, generating NADH. Subsequent steps involve substrate-level phosphorylation, where phosphate groups are transferred to ADP to form ATP. The final step, catalyzed by pyruvate kinase, converts phosphoenolpyruvate to pyruvate. This step is irreversible and provides the second site of substrate-level phosphorylation. In the absence of oxygen, the NADH produced must be recycled back to NAD+ by converting pyruvate to lactate, a process essential for maintaining the redox balance required for continued glycolytic flux.

Glycolysis is central to understanding metabolic acidosis, particularly lactic acidosis (Type A due to hypoxia, Type B due to metabolic failure). Pyruvate kinase deficiency is the second most common cause of enzyme-deficient haemolytic anaemia; RBCs depend entirely on glycolysis for ATP to maintain the Na+/K+ ATPase pump, and deficiency leads to cell rigidity and splenic sequestration. The 'Warburg Effect' describes how cancer cells preferentially use glycolysis even in the presence of oxygen, a principle utilized in PET imaging with fluorodeoxyglucose (FDG). Understanding this pathway is also vital for managing diabetes mellitus, where insulin deficiency or resistance disrupts the regulation of glucokinase and PFK-1, leading to hyperglycaemia.

Investigations related to glycolytic dysfunction include: Serum Lactate (elevated in hypoxia or sepsis), Blood Glucose (monitoring metabolic control), and Full Blood Count (checking for anaemia in pyruvate kinase deficiency). Enzyme assays for pyruvate kinase activity may be performed in specialized labs. Arterial Blood Gas (ABG) is indicated to evaluate pH and bicarbonate levels in suspected lactic acidosis.

Acute management of lactic acidosis involves treating the underlying cause (e.g., oxygenation, fluid resuscitation for sepsis). In chronic conditions like pyruvate kinase deficiency, management includes folate supplementation, blood transfusions, or splenectomy in severe cases. Nutritional management of diabetes involves balancing carbohydrate intake with insulin to regulate glycolytic flux.