The ten reactions of glycolysis, or the breakdown of glucose into pyruvate, occurs in two stages.
The first five reactions correspond to an energy investment of two ATP, while the last five produce 4 ATP for a net amount of 2 ATP per glucose. The reactions of Hexokinase, PFK-1, and pyruvate kinase are irreversible.
Phosphorylation of glucose to glucose-6-phosphate occurs through the action of various isoforms of hexokinase, which has a low Km (and therefore high affinity) for glucose. This also means that it has a low VMax, however.
In the liver and pancreatic islet cells, glucokinase is the predominant enzyme that phosphorylates glucose. Glucokinase has a high Km and VMax, preventing it from working at low glucouse concentrations but allowing it to respond to a flood of glucose, minimizing systemic hyperglycemia.
Phosphorylation of fructose-6-phosphate to fructose-1,6-bis P is irreversible and catalyzed by phosphofructokinase-1 (PFK-1). This rate-limiting step is the most important control point of glycolysis.
Cleavage of fructose 1,6-bisphosphate yields a molecule each of glyceraldehyde-3-P and DHAP, which is converted into glyceraldehyde-3-P.
Oxidation of glyceraldehyde-3-phosphate results in 1,3-BPG, which can be converted to 2,3-BPG, an allosteric controller of hemoglobin affinity for oxygen.
A few other steps occur to produce phosphoenolpyruvate (PEP).
PEP is converted to pyruvate through the irreversible action of pyruvate kinase, generating ATP.
Pyruvate can then go on to enter the TCA Cycle, or can by converted into lactate.
Reduction of pyruvate to lactate, but lactate dehydrogenase, is the last step of anaerobic glycolysis. Lactate is the major fate of pyruvate in RBCs, the eye, the kidney medulla, testes, and leukocytes. RBCs produce the most lactate per day (30g) followed by skin, brain, skeletal muscle, and renal medulla.
An elevated NADH/NAD+ ratio in skeletal muscle can occur during intense exercise due to increased activity of glyceraldehyde 3-P dehydrogenase and production of NADH that exceeds the capacity of the TCA cycle. This increased NADH favors lactate production, leading to a drop in pH and increased release of oxygen in muscles. Much of this lactate diffuses into the blood and can be used in the liver for gluconeogenesis.
Elevated lactate levels are present in the plasma during circulatory collapse, as can occur during an MI, a PE, uncontrolled hemmorage, or shock. Execess oxygen is required to require from a time of low oxygen, and this amount is called the oxygen debt. The level of oxygen debt has been related to morbity and mortality.
In the liver, glucokinase is normally sequestered on the nucleus. Increasing intracellular glucose facilitates release of glucokinase into the cytoplasm. As glucose levels fall, fructose-6-phosphate causes glucokinase to translocate back to the nucleus and become inactive. Glucokinase transcription is increased by insulin.
PFK-1 is activated by fructose 2,6-bisphosphate, which is synthesized by PFK-2. F-2,6-bisP is increased by a high insulin/glucagon ratio, activating PKA and PFK-2. It is also stimulated by AMP.
In cardiac muscle, norephinephrine activates PFK-2, while in skeletal muscle, fructose-6-P activates it. In liver, a high insulin/glucagon ratio activates PFK-2, which a low ratio inhibits it.
PFK-1 is inhibited by high levels of ATP and citrate, an intermediate in the TCA cycle. The levels of ATP in cardiac muscle would inhibit PFK-1 were it not for the presence of AMP and F-2,6-bisP.
F-2,6-bisP also acts to inhibit fructose 1,6-bisphosphatase in the gluconeogenesis pathway. This prevents both pathways from being active at the same time.