Glycolysis

Glycolysis, TCA Cycle and Energy Metabolism

Glycolysis converts glucose into pyruvate in the cytoplasm. It can generate ATP without directly requiring oxygen, although the fate of pyruvate depends on cellular conditions.

Main steps and enzymes

Step	Enzyme	High-yield point
Glucose → glucose-6-phosphate	Hexokinase/glucokinase	ATP is consumed
Fructose-6-phosphate → fructose-1,6-bisphosphate	PFK-1	Rate-limiting, committed step
Fructose-1,6-bisphosphate → triose phosphates	Aldolase	Produces two 3-carbon intermediates
Glyceraldehyde-3-phosphate → 1,3-bisphosphoglycerate	GAP dehydrogenase	NADH generated
1,3-Bisphosphoglycerate → 3-phosphoglycerate	Phosphoglycerate kinase	ATP generated
Phosphoenolpyruvate → pyruvate	Pyruvate kinase	ATP generated

Net yield per glucose

Product	Net yield
ATP	2
NADH	2
Pyruvate	2

Two ATP are consumed in the early phase, and four ATP are produced later, giving a net yield of two ATP per glucose.

Rate-limiting enzyme: PFK-1

PFK-1 is a major regulatory point in glycolysis.

Activated by AMP and fructose-2,6-bisphosphate.
Inhibited by ATP and citrate.
Fructose-2,6-bisphosphate is an important signal favoring glycolysis in the fed state.

Fate of pyruvate

Condition/pathway	Product	Purpose
Aerobic metabolism	Acetyl-CoA	Enters the TCA cycle
Anaerobic glycolysis	Lactate	Regenerates NAD⁺ so glycolysis can continue
Gluconeogenesis in appropriate tissues	Oxaloacetate-related intermediates	Supports glucose synthesis
Amino acid metabolism	Alanine through transamination	Links carbohydrate and nitrogen metabolism

TCA Cycle

The tricarboxylic acid (TCA) cycle occurs primarily in the mitochondrial matrix and oxidizes acetyl-CoA, generating reduced cofactors for oxidative phosphorylation.

Major enzymes and products

Enzyme	Reaction/high-yield association
Citrate synthase	Acetyl-CoA + oxaloacetate → citrate
Isocitrate dehydrogenase	Produces CO₂ and NADH; important regulatory step
α-Ketoglutarate dehydrogenase	Produces CO₂ and NADH; requires TPP, lipoate, FAD, NAD⁺ and CoA
Succinyl-CoA synthetase	Produces GTP or ATP, depending on tissue/isoenzyme
Succinate dehydrogenase	Produces FADH₂; also Complex II of the electron transport chain
Malate dehydrogenase	Regenerates oxaloacetate and produces NADH

Yield per acetyl-CoA

Product	Quantity
NADH	3
FADH₂	1
GTP	1
CO₂	2

Using approximate modern ATP equivalents, one acetyl-CoA yields about 10 ATP through oxidative phosphorylation and substrate-level phosphorylation.

Oxidative Phosphorylation and ATP

Electrons from NADH and FADH₂ pass through the mitochondrial electron transport chain. The energy released pumps protons across the inner mitochondrial membrane, creating a gradient that drives ATP synthesis.

Complex	Main role	Important fact
I	NADH dehydrogenase	Transfers electrons from NADH; pumps protons
II	Succinate dehydrogenase	Transfers electrons from FADH₂-linked reactions; does not pump protons
III	Cytochrome bc₁ complex	Transfers electrons toward cytochrome c; pumps protons
IV	Cytochrome c oxidase	Transfers electrons to O₂, the final electron acceptor
V	ATP synthase	Uses the proton gradient to synthesize ATP

ATP yield

Modern estimates commonly use:

NADH: approximately 2.5 ATP.
FADH₂: approximately 1.5 ATP.

Complete oxidation of one glucose molecule generally yields about 30–32 ATP, depending on the shuttle system and cellular context.

High-yield glycolysis versus TCA distinctions

Feature	Glycolysis	TCA cycle
Location	Cytoplasm	Mainly mitochondrial matrix
Direct oxygen requirement	No	Depends indirectly on oxidative metabolism to regenerate cofactors
Major output	Pyruvate, ATP, NADH	NADH, FADH₂, GTP, CO₂
Main regulatory enzyme	PFK-1	Isocitrate dehydrogenase is a major regulatory step
Anaerobic ATP production	Yes	Does not sustain normal flux without regenerated cofactors

Exam tip: Succinate dehydrogenase is the only TCA-cycle enzyme embedded in the inner mitochondrial membrane and is also Complex II.