Glycolysis for FMGE: The Three Enzymes That Actually Matter

By Dr. Utsav Bhattacherjee, MBBS, MBA · 17 September 2026 · 11 min read

The Two Phases: Investment, Then Payoff

Glycolysis splits cleanly into two phases. The investment phase consumes 2 ATP to convert one glucose molecule into two molecules of glyceraldehyde-3-phosphate — an energy cost paid upfront to destabilise the glucose molecule enough for the pathway to proceed. The payoff phase then generates 4 ATP and 2 NADH from those two three-carbon molecules, for a net yield of 2 ATP and 2 NADH per glucose molecule overall.

The Ten Steps, at a Glance

The Three Irreversible Steps

Three reactions in glycolysis are irreversible under physiological conditions, and all three are catalysed by regulated, rate-limiting enzymes — these three are where the exam’s actual attention concentrates, far more than the reversible steps in between.

Hexokinase (or Glucokinase in the Liver)

The first committed step traps glucose inside the cell by phosphorylating it to glucose-6-phosphate, a molecule that can no longer cross the cell membrane freely. Hexokinase, present in most tissues, has a low Km (high affinity for glucose) and is inhibited by its own product, glucose-6-phosphate — a direct feedback brake. Glucokinase, the liver-specific isoform, has a much higher Km, is not inhibited by glucose-6-phosphate, and only becomes active once blood glucose is genuinely high — which is exactly why the liver only takes up large amounts of glucose for storage after a meal, rather than competing with other tissues for glucose when levels are normal or low.

Phosphofructokinase-1 (PFK-1): The Real Control Point

PFK-1 catalyses the single most important regulated step in the entire pathway, and it’s the step examiners return to most often. PFK-1 is allosterically activated by AMP and fructose-2,6-bisphosphate (a signal that energy is needed) and inhibited by ATP and citrate (a signal that energy and biosynthetic intermediates are already abundant). This single enzyme is effectively the pathway’s throttle — everything upstream of it can proceed regardless, but flux through glycolysis as a whole is set here.

Pyruvate Kinase: The Final Committed Step

Pyruvate kinase catalyses the last step, generating ATP directly and committing the pathway’s product to pyruvate. It’s activated by fructose-1,6-bisphosphate (feed-forward activation, meaning an upstream intermediate accelerates a downstream step) and inhibited by ATP and alanine — alanine inhibition is a genuinely useful fact to hold onto, since it links glycolysis directly to amino acid metabolism and the glucose-alanine cycle between muscle and liver.

How Insulin and Glucagon Set the Pathway's Overall Direction

PFK-1’s regulation by fructose-2,6-bisphosphate is itself under hormonal control, which is what connects this one enzyme to whole-body glucose homeostasis rather than just local energy charge. Insulin, released when blood glucose is high, raises fructose-2,6-bisphosphate levels, activating PFK-1 and pushing flux through glycolysis forward. Glucagon, released when blood glucose is low, lowers fructose-2,6-bisphosphate, easing off PFK-1 and favouring the reverse process, gluconeogenesis, instead. This is the mechanistic link between a hormone released in response to a meal and an increase in glucose being broken down for energy at the cellular level.

Where NADH Actually Comes From

The single NADH-generating step in glycolysis is glyceraldehyde-3-phosphate dehydrogenase, which oxidises glyceraldehyde-3-phosphate while reducing NAD+ to NADH. Since this step occurs twice per glucose molecule (once for each three-carbon fragment), the pathway generates 2 NADH total per glucose — and regenerating that NAD+ supply is the entire reason fermentation (lactate or ethanol production) exists under anaerobic conditions: without regenerating NAD+, glycolysis itself would grind to a halt for lack of the oxidised cofactor this one step depends on.

What Happens to Pyruvate Next

Glycolysis itself ends at pyruvate, but where pyruvate goes next depends entirely on oxygen availability, and knowing this branch point matters for placing glycolysis correctly within the larger picture of energy metabolism. Under aerobic conditions, pyruvate is transported into the mitochondria and converted to acetyl-CoA by pyruvate dehydrogenase, feeding into the citric acid cycle and, ultimately, oxidative phosphorylation — this route extracts far more ATP per glucose molecule than glycolysis alone, but requires oxygen as the final electron acceptor at the end of the chain.

Under anaerobic conditions, pyruvate is instead reduced to lactate (in most human tissues) or, in yeast, decarboxylated and reduced to ethanol — both routes exist for the same single purpose already discussed: regenerating NAD+ so glycolysis can continue without a functioning electron transport chain. The ATP yield from glycolysis alone, 2 per glucose, is a small fraction of what aerobic metabolism eventually extracts from the same starting molecule, which is exactly why tissues capable of aerobic metabolism strongly prefer it whenever oxygen is available.

This branch point is often where a glycolysis question actually tests understanding rather than recall: describing a tissue or condition (red blood cells, intense exercise, an anaerobic environment) and asking what happens to pyruvate is testing whether the oxygen-dependence of the branch is understood, not whether the ten steps of glycolysis itself can be recited.

Why Anaerobic Glycolysis Still Matters

Red blood cells, lacking mitochondria entirely, depend on anaerobic glycolysis as their only source of ATP, converting pyruvate to lactate specifically to regenerate NAD+ and keep the pathway running. Skeletal muscle during intense exertion does the same thing when oxygen delivery can’t keep pace with demand — the resulting lactate is not simply metabolic waste, it’s shuttled to the liver via the Cori cycle and converted back to glucose through gluconeogenesis, making the whole system a genuine cycle rather than a dead end.

Cancer cells offer a genuinely interesting twist on this same branch point, worth knowing as an aside: many tumour cells preferentially ferment glucose to lactate even when oxygen is fully available, a phenomenon known as the Warburg effect. The exact reason cancer cells favour this apparently less efficient route despite having oxygen available is still debated, but it is thought to relate to the biosynthetic demands of rapidly dividing cells, which need glycolytic intermediates as building blocks for nucleotides, amino acids, and lipids more than they need the maximal ATP yield aerobic metabolism would otherwise provide, and this preference is measurable enough to be exploited diagnostically in PET imaging of tumours.

How Glycolysis Is Tested on FMGE

Beyond the regulated enzymes, two inhibitors come up often enough to be worth knowing by name. Fluoride inhibits enolase, which is the reason a blood sample drawn for glucose estimation goes into a fluoride-containing tube — glycolysis in the sample would otherwise keep consuming glucose after collection and return a falsely low result. That is a laboratory fact with a biochemical explanation, and it is asked from both directions. Arsenate is the second: it substitutes for inorganic phosphate at the glyceraldehyde-3-phosphate dehydrogenase step, and the product hydrolyses spontaneously instead of transferring its phosphate to ADP. The pathway keeps running while the ATP that step should have yielded is simply lost — worth distinguishing from arsenite, which acts on pyruvate dehydrogenase downstream rather than on glycolysis itself.

The red cell is the other reliable source of questions, because it depends on this pathway completely. Mature red cells divert some 1,3-bisphosphoglycerate through the Rapoport-Luebering shunt to make 2,3-bisphosphoglycerate, which binds deoxyhaemoglobin and shifts the oxygen dissociation curve to the right, releasing oxygen more readily to the tissues. The cell pays for it by giving up the ATP that step would otherwise have generated — a genuine trade of energy for oxygen delivery, and the reason glycolysis questions and haemoglobin questions sometimes turn out to be the same question.

Working through FMGE previous year questions shows how narrow the tested range actually is — the regulated enzymes, these inhibitors, and the red cell account for most of it. For the subject around it, how to prepare Biochemistry for FMGE sets out how the metabolic pathways are weighted against the rest, and the FMGE Biochemistry subject page collects the related topics.

A Clinical Correlation Worth Knowing: Pyruvate Kinase Deficiency

Pyruvate kinase deficiency is the classic clinical correlation for this pathway, and it’s specifically tested through its effect on red blood cells. Since mature red blood cells have no mitochondria and depend entirely on glycolysis for ATP, a deficiency in the pathway’s final enzyme leaves red cells without enough ATP to maintain their membrane integrity and normal biconcave shape, leading to hemolytic anemia. This is a genuinely useful example of why glycolysis, often treated as a purely biochemical pathway, has a direct and testable clinical consequence when one of its enzymes fails.

Reading a glycolysis question efficiently means checking, first, whether it is asking about a regulated step (hexokinase, PFK-1, or pyruvate kinase) or a connecting step in between — the regulated steps carry almost all of the pathway’s clinical and biochemical significance, and are where exam attention concentrates far more heavily than the reversible reactions linking them together.