Glycolysis

Glycolysis

A catabolic pathway that converts one molecule of glucose into two molecules of pyruvate, generating ATP and NADH.

Conditions for glycolysis

It occurs in the cytosol and can function under both aerobic and anaerobic conditions.

Hexokinase

An enzyme that catalyzes the phosphorylation of glucose to glucose-6-phosphate using ATP. This is an irreversible and regulated step in glycolysis.

Phosphofructokinase-1 (PFK-1)

The enzyme that catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate. It is tightly regulated and considered the committed step in glycolysis.

Pyruvate Kinase

An enzyme that catalyzes the conversion of phosphoenolpyruvate to pyruvate, generating ATP. This step is irreversible and regulated.

Glucose-6-Phosphate

An intermediate in glycolysis formed by the action of hexokinase. It is a branching point for multiple metabolic pathways.

Fructose-1,6-bisphosphate

A glycolytic intermediate formed from fructose-6-phosphate by the action of PFK-1. It is also an allosteric activator of pyruvate kinase.

Glyceraldehyde-3-phosphate Dehydrogenase (GAPDH)

An enzyme that catalyzes the oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, producing NADH. This is the first oxidation reaction in glycolysis.

1,3-Bisphosphoglycerate

A high-energy intermediate in glycolysis that participates in substrate-level phosphorylation to generate ATP.

Phosphoenolpyruvate (PEP)

A high-energy intermediate in glycolysis that donates a phosphate to ADP to form ATP in a reaction catalyzed by pyruvate kinase.

Substrate-Level Phosphorylation

The direct synthesis of ATP from ADP by a high-energy substrate, independent of the electron transport chain.

Anaerobic Glycolysis

The process of glycolysis occurring in the absence of oxygen, leading to the production of lactate from pyruvate and regeneration of NAD+.

Lactate

A product of anaerobic glycolysis formed from pyruvate. Its production regenerates NAD+ and helps maintain glycolysis under hypoxic conditions.

Pyruvate Dehydrogenase Complex (PDC)

A mitochondrial enzyme complex that converts pyruvate to acetyl-CoA, linking glycolysis to the citric acid cycle. It is irreversible and regulated.

Acetyl-CoA

A two-carbon molecule derived from pyruvate via the PDC. It enters the citric acid cycle and is a substrate for energy production.

Thioester Bond

A high-energy bond between a carboxylic acid and a thiol group, such as in acetyl-CoA. Breaking this bond releases energy.

Feedforward Activation

A regulatory mechanism where a metabolite early in a pathway (e.g., fructose-1,6-bisphosphate) activates an enzyme later in the pathway (e.g., pyruvate kinase).

Allosteric Regulation

Regulation of an enzyme by binding of an effector molecule at a site other than the enzyme's active site, affecting its activity.

Irreversible Reaction

A reaction that proceeds in one direction under physiological conditions and often serves as a control point in metabolic pathways.

Gluconeogenesis

The anabolic, opposing pathway to glycolysis, synthesizing glucose from non-carbohydrate sources. It shares several enzymes with glycolysis but includes bypass reactions due to irreversible glycolytic steps.

Fructose-6-phosphate (F6P)

An intermediate in glycolysis formed from glucose-6-phosphate through isomerization.

Fructose-1,6-bisphosphate (F-1,6-BP)

Formed from F6P by PFK-1. It is split into DHAP and GAP in glycolysis.

Dihydroxyacetone phosphate (DHAP)

An isomer of GAP formed from F-1,6-BP. It can be converted to GAP to continue glycolysis.

Glyceraldehyde-3-phosphate (GAP)

A 3-carbon sugar that proceeds through the energy payout phase of glycolysis after formation from F-1,6-BP.

3-Phosphoglycerate

Formed from 1,3-BPG by phosphoglycerate kinase. ATP is generated in this substrate-level phosphorylation step.

ATP

Generated in the substrate-level phosphorylation step.

2-Phosphoglycerate

Formed from 3-phosphoglycerate by phosphoglycerate mutase. Precursor to PEP.

Phosphoglycerate Kinase

Catalyzes the substrate-level phosphorylation of ADP using 1,3-BPG to form ATP and 3-phosphoglycerate.

Phosphoglycerate Mutase

Catalyzes the reversible conversion of 3-phosphoglycerate to 2-phosphoglycerate.

Enolase

Catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP), a high-energy intermediate.

NAD+/NADH

NAD+ is an oxidizing agent accepting electrons in glycolysis to form NADH, which carries electrons to the electron transport chain.

ATP Investment Phase

The first phase of glycolysis where 2 ATP are consumed to activate glucose and F6P.

ATP Payout Phase

The second phase of glycolysis where 4 ATP are generated per glucose, yielding a net gain of 2 ATP.

Isomerization

Reversible conversion of one molecule into another isomer, such as glucose-6-phosphate to fructose-6-phosphate.

Lysis

Cleavage of a molecule, such as splitting of F-1,6-BP into DHAP and GAP in glycolysis.

Oxidation

Loss of electrons from a molecule, such as GAP being oxidized to 1,3-BPG by GAPDH.

Dehydration

Removal of water from a molecule, as in the conversion of 2-phosphoglycerate to PEP by enolase.

Pyruvate

End product of glycolysis. Under aerobic conditions, converted to acetyl-CoA; under anaerobic conditions, reduced to lactate.

Pyruvate Translocase

A transporter that moves pyruvate from the cytosol into the mitochondrial matrix using a proton symport mechanism.

Lactate Dehydrogenase (LDH)

Enzyme that catalyzes the reversible conversion of pyruvate to lactate, regenerating NAD+ during anaerobic glycolysis.

Cori Cycle

Metabolic pathway where lactate produced in muscles is transported to the liver, converted to glucose, and sent back to muscles.

Glycogenolysis

The breakdown of glycogen into glucose-1-phosphate, which is converted to G6P and enters glycolysis.

Glucose Transporters (GLUTs)

Proteins facilitating glucose entry into cells. Different tissues express different isoforms with varying affinities.

Phosphofructokinase-1 (PFK-1)

Catalyzes the rate-limiting, irreversible phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate in glycolysis. It is allosterically activated by ADP/AMP and inhibited by ATP and PEP.

Feedforward Activation

A regulatory mechanism where an early metabolite (e.g., fructose-1,6-bisphosphate) activates a downstream enzyme (e.g., pyruvate kinase).

Phosphoanhydride Bond

A high-energy bond found in ATP that releases significant energy upon hydrolysis (~ -32 kJ/mol).

Pyruvate Kinase Regulation

Regulated allosterically by fructose-1,6-bisphosphate (activator) and ATP (inhibitor). Plays a role in reciprocal regulation with gluconeogenesis.

GAPDH Reaction

Step 6 in glycolysis. GAP is oxidized and phosphorylated by GAPDH to form 1,3-BPG, producing NADH.

2,3-Bisphosphoglycerate (2,3-BPG)

A molecule derived from 1,3-BPG in erythrocytes; binds hemoglobin to reduce its oxygen affinity and enhance oxygen unloading.

Thioester Bond

A high-energy bond between a thiol and a carboxylic acid, such as in acetyl-CoA. Used to activate acetate and other acyl groups.

Phosphorylation States

Glycolysis uses phosphorylation and dephosphorylation reactions to control energy investment and capture. Phosphorylated intermediates ensure pathway directionality and enzyme regulation.

Lactate Export

Lactate is exported from muscle cells via a plasma membrane symporter with H+, lowering intracellular lactate and enabling continued NAD+ regeneration.

Pyruvate Dehydrogenase Complex (PDC) Regulation

Regulated by reversible phosphorylation: kinase inactivates, phosphatase activates. Also regulated by energy levels (ATP, NADH, Acetyl-CoA).

Multienzyme Complex

Pyruvate Dehydrogenase Complex is a multienzyme structure that improves reaction speed and specificity by channeling intermediates between enzyme active sites.