Metabolic pathways

Metabolism

The process in organisms that involves the liberation and storage of energy from foods and the creation of complex molecules from simpler ones through enzyme-catalyzed reactions of both matter and energy.

Metabolic pathway

A sequence of enzymatic reactions where the product of one reaction becomes the substrate of the next reaction, resulting in the formation of metabolites (metabolic intermediates).

Catabolism

The degradative process in metabolism where complex molecules are broken down into simpler ones.

Anabolism

The biosynthesis process in metabolism where building block compounds are transformed into more complex molecules, requiring energy through ATP.

Glycolysis

The major pathway of glucose utilization in the cytosol, involving the breakdown of glucose to provide energy and intermediates for other metabolic pathways.

Energy investment reactions

Reactions in glycolysis that consume ATP.

Energy recovery reactions

Reactions in glycolysis that produce ATP.

Na+-dependent monosaccharide cotransport system

A transport system that allows the uptake of D-glucose and D-galactose into cells using the energy of the Na+ gradient.

Na+-independent system

A transport system that facilitates the uptake of D-fructose through facilitated diffusion.

Phosphorylation of glucose

The addition of a phosphate group to glucose, catalyzed by hexokinase enzymes, resulting in the formation of glucose-6-phosphate.

Isomerization of G-6-P

The conversion of glucose-6-phosphate into fructose-6-phosphate, catalyzed by phosphoglucose isomerase.

Phosphorylation of F-6-P

The addition of a phosphate group to fructose-6-phosphate, catalyzed by phosphofructokinase-1, resulting in the formation of fructose 1,6-bisphosphate.

Cleavage of F-1,6-bisP

The hydrolysis of fructose 1,6-bisphosphate into two 3-carbon products, dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P), catalyzed by aldolase.

Isomerization of triose phosphates

The reversible conversion of DHAP into G3P, catalyzed by triosephosphate isomerase.

Oxidation of glyceraldehyde 3-phosphate (GAP)

The NAD+-dependent oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate, catalyzed by glyceraldehyde-3-P dehydrogenase.

Formation of ATP from 1,3-bisphosphoglycerate and ADP

The conversion of 1,3-bisphosphoglycerate into ATP and 3-phosphoglycerate, catalyzed by phosphoglycerate kinase.

Isomerization of 3-P-glycerate to 2-p-glycerate

The conversion of 3-phosphoglycerate into 2-phosphoglycerate, catalyzed by phosphoglycerate mutase.

Dehydration of 2-P-glycerate

The conversion of 2-phosphoglycerate into phosphoenolpyruvate, catalyzed by enolase.

Formation of pyruvate

The conversion of phosphoenolpyruvate into pyruvate, catalyzed by pyruvate kinase.

Reduction of pyruvate to lactate

The reduction of pyruvate to lactate, catalyzed by lactate dehydrogenase, which occurs in the final part of anaerobic glycolysis.

Pyruvate dehydrogenase complex

A complex of enzymes that converts pyruvate into acetyl-CoA, which enters the Krebs cycle.

Krebs cycle

A pathway for the oxidation of fuel molecules, including sugars, amino acids, and fatty acids, in the mitochondrial matrix, generating reduced coenzymes (NADH and FADH2) and high-energy compounds (GTP).

Citrate synthase

An enzyme that catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate in the Krebs cycle.

Isocitrate dehydrogenase

An enzyme that catalyzes the oxidative decarboxylation of isocitrate in the Krebs cycle.

α-ketoglutarate dehydrogenase

An enzyme that catalyzes the oxidative decarboxylation of α-ketoglutarate in the Krebs cycle.

Succinyl-CoA

A high-energy compound formed in the Krebs cycle that

Electron transport chain

A system of oxidoreductases that passes electrons from NADH and FADH2 to oxygen.

Mitochondrion

The organelle responsible for generating energy in the form of ATP.

Oxidative phosphorylation

The process that couples respiration and ATP synthesis.

Outer membrane

The phospholipid bilayer of the mitochondrion that is permeable to small molecules.

Inner membrane

The highly folded membrane of the mitochondrion that contains the electron transport chain and ATP synthetase complex.

Matrix

The compartment within the mitochondrion that contains enzymes responsible for the citric acid cycle and fatty acid oxidation.

Electron carriers

Coenzyme Q and cytochrome C, which transfer electrons between the complexes of the electron transport chain.

Complex I

NADH dehydrogenase, responsible for transferring electrons from NADH to CoQ.

Complex II

Succinate dehydrogenase, transfers electrons from succinate to CoQ.

CoQ (ubiquinone)

A small mobile carrier that transfers electrons between complex I or II and cytochrome b.

Complex III

Cytochromes bc1, contains heme proteins and transfers electrons from CoQ to cytochrome c.

Cytochrome c

The smallest electron carrier that transfers electrons from complex III to complex IV.

Complex IV

Cytochrome oxidase, contains hemoproteins and oxidizes Cu+ to Cu2+ during electron transfer to oxygen.

Proton motive force

The electrochemical gradient created by the difference in pH and charge across the inner mitochondrial membrane.

ATP synthase

A complex that uses the proton motive force to synthesize ATP from ADP and inorganic phosphate.

P/O ratio

The ratio of inorganic phosphates converted to ATP when 1 atom of O2 is consumed.

Uncouplers

Small molecules that increase the permeability of the inner mitochondrial membrane to protons, uncoupling electron transport from ATP synthesis.

Malate-aspartate shuttle

A shuttle that produces NADH in the mitochondrial matrix, yielding three ATP for each cytosolic NADH oxidized.

Glycogen

A branched chain homopolysaccharide of glucose that serves as a fuel reserve for ATP synthesis.

Glycogenolysis

The degradation of glycogen by glycogen phosphorylase, resulting in the production of glucose-1-phosphate.

Glycogen phosphorylase

Enzyme that degrades glycogen chains until 4 glycosyl units remain on each chain before a branch point.

Limit dextrin

Resulting structure after glycogen phosphorylase degrades glycogen chains until 4 glycosyl units remain on each chain before a branch point.

Debranching enzyme

Enzyme that removes branch points in glycogen by transferring outer 3 of 4 glycosyl residues to the end of the chain and hydrolytically removing the remaining single glucose residue.

Glucose-1-phosphate

Converted to glucose-6-phosphate by phosphoglucomutase and is present in liver and muscle.

Glucose-6-phosphatase

Enzyme present in liver cells that enables glucose to leave the liver, kidney, and intestine but not muscles.

Glycogen synthase

Enzyme that synthesizes glycogen from excess glucose in the cytosol using ATP and two substrates:UDP and glycogen molecule.

Primer

Fragment of glycogen that serves as a primer in cells whose glycogen stores are not totally depleted.

Glycogen branches

Produced by branching enzymes, which transfer a terminal fragment of 6-7 glucose residues from at least 11 glucose residues and are further elongated by glycogen synthase.

Glycogen synthase and glycogen phosphorylase

Allosterically controlled enzymes in glycogen metabolism, with glycogen synthase being activated by G-6-P and glycogen phosphorylase being inhibited by G-6-P and ATP.

cAMP pathway

Hormone regulation of glycogen metabolism through the formation of cAMP, which activates protein kinase A (PKA) to phosphorylate and activate glycogen phosphorylase and phosphorylate and inactivate glycogen synthase.

Gluconeogenesis

Biosynthesis of new glucose from other metabolites, mainly occurring in the liver and kidney to maintain constant blood glucose concentration.

Pyruvate

Major precursor for gluconeogenesis, converted into glucose through a series of reactions including carboxylation and decarboxylation.

Lactate

Released into the blood by cells lacking mitochondria and converted into pyruvate in the liver for gluconeogenesis.

Glycerol

Converted into glycerol-3-phosphate and further into fructose-1,6-bisphosphate for gluconeogenesis.

Amino acids

Degraded to Krebs cycle intermediates, with their carbon skeletons converted to pyruvate for gluconeogenesis.

Pentose phosphate pathway

Produces NADPH and ribose 5-phosphate for nucleotide synthesis, and converts glucose into other sugars, mainly active in tissues involved in cholesterol and fatty acid synthesis.

Fructose metabolism

Enters glycolysis as GAP and DAP through a series of reactions in the liver or through the action of hexokinase in other tissues.

Galactose metabolism

Involves the conversion of galactose to glucose-1-phosphate through a series of reactions.

Glucose-6-phosphate (G-6-P)

A molecule formed through a series of five enzymatic steps from glucose.

Enzymatic steps

A series of chemical reactions catalyzed by enzymes that convert glucose into G-6-P.

Glucose

A simple sugar and the primary source of energy for cells.

Phosphate

A chemical group consisting of a phosphorus atom bonded to four oxygen atoms, often involved in energy transfer reactions.

Conversion

The process of changing one form or substance into another, in this case, the transformation of glucose into G-6-P.