22.4 Glycolysis: Oxidation of Glucose Glucose in the bloodstream enters our cells, where it undergoes degradation in a pathway called glycolysis. Glucose obtained from the digestion of polysaccharides is degraded in glycolysis to pyruvate. Learning Goal Describe the conversion of glucose to pyruvate in glycolysis.

Glycolysis Glycolysis takes place in the cytosol of the cell. is a metabolic pathway that uses glucose, a digestion product. degrades six-carbon glucose molecules to three-carbon pyruvate molecules. Core Chemistry Skill Identifying the Compounds in Glycolysis

Glycolysis: Energy Investment In reactions 1 to 5 of glycolysis, energy is required to add phosphate groups to glucose. glucose is converted to two three-carbon molecules.

Glycolysis: Energy Generating In reactions 6 to 10 of glycolysis, energy is obtained from the hydrolysis of the energy-rich phosphate compounds. four ATP are synthesized.

Glycolysis: Reaction 1 In reaction 1, phosphorylation, a phosphate group is transferred from ATP to glucose. glucose-6-phosphate and ADP are produced. the enzyme hexokinase catalyzes the reaction.

Glycolysis: Reaction 2 In reaction 2, isomerization, glucose-6-phosphate, the aldose from reaction 1, is converted to fructose-6-phosphate. the isomerization is catalyzed by the enzyme phosphoglucose isomerase.

Glycolysis: Reaction 3 In reaction 3, phosphorylation, hydrolysis of another ATP provides a second phosphate group. the phosphate group is transferred to fructose-6-phosphate, producing fructose-1,6-bisphosphate. a second kinase enzyme called phosphofructokinase catalyzes the reaction.

Glycolysis: Reaction 4 In reaction 4, cleavage, fructose-1,6-bisphosphate is split into two three-carbon phosphate isomers. the enzyme aldolase produces dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.

Glycolysis: Reaction 5 In reaction 5, isomerization, dihydroxyacetone phosphate undergoes isomerization catalyzed by triose phosphate isomerase. a second molecule of glyceraldehyde-3-phosphate is produced, which can be oxidized. all six carbon atoms from glucose are contained in two identical triose phosphates.

Glycolysis: Reaction 6 In reaction 6, oxidation and phosphorylation, the aldehyde group of each glyceraldehyde-3-phosphate is oxidized to a carboxyl group. NAD+ is reduced to NADH and H+. a phosphate group is transferred to each of the new carboxyl groups, forming two molecules of 1,3-bisphosphoglycerate.

Glycolysis: Reaction 7 In reaction 7, phosphate transfer, a phosphate group from each 1,3-bisphosphoglycerate is transferred to two ADP molecules by phosphoglycerate kinase. two molecules of the high-energy compound ATP are produced.

Glycolysis: Reaction 8 In reaction 8, isomerization, two 3-phosphoglycerate molecules undergo isomerization by phosphoglycerate mutase. the phosphate group is moved from carbon 3 to carbon 2, yielding two molecules of 2-phosphoglycerate.

Glycolysis: Reaction 9 In reaction 9, dehydration, each phosphoglycerate molecule undergoes dehydration by the enzyme enolase. two high-energy phosphoenolpyruvate molecules are produced.

Glycolysis: Reaction 10 In reaction 10, phosphate transfer, phosphate groups from two phosphoenolpyruvate molecules are transferred by pyruvate kinase to yield two ADP, two pyruvate, and two ATP. a fourth kinase enzyme transfers a phosphate with ATP production.

Glycolysis: Overall Reaction In glycolysis, two ATP add phosphate to glucose and fructose-6-phosphate. four ATP are formed in energy generation by direct transfers of phosphate groups to four ADP. there is a net gain of 2 ATP and 2 NADH.

Fructose and Galactose Other monosaccharides, such as fructose and galactose, can enter glycolysis after they are converted to intermediates. In muscles and kidneys, fructose is phosphorylated to fructose-6-phosphate, which enters glycolysis in reaction 3. Galactose reacts with ATP to yield galactose-1-phosphate, which is converted to glucose-6-phosphate, which then enters glycolysis at reaction 2.

Fructose and Galactose Galactose and fructose form intermediates that enter the glycolysis pathway to be metabolized.

Regulation of Glycolysis Glycolysis is regulated by three enzymes. In reaction 1, hexokinase is inhibited by high levels of glucose-6-phosphate, which prevents the phosphorylation of glucose. In reaction 3, phosphofructokinase, an allosteric enzyme, is inhibited by high levels of ATP and activated by high levels of ADP and AMP. In reaction 10, pyruvate kinase, another allosteric enzyme, is inhibited by high levels of ATP or acetyl CoA.

Study Check In glycolysis, what compounds provide phosphate groups for the production of ATP?

Solution In glycolysis, what compounds provide phosphate groups for the production of ATP? In reaction 7, phosphate groups from two 1,3-bisphosphoglycerate molecules are transferred to ADP to form two ATP. In reaction 10, phosphate groups from two phosphoenolpyruvate molecules are used to form two more ATP.

The Pentose Phosphate Pathway is an alternative pathway for the oxidation of glucose. produces the coenzyme NADPH and five-carbon pentoses. NADPH, the reduced form of NADP+, is an important coenzyme required in the anabolic pathways, including the biosynthesis of nucleic acids, cholesterol, and fatty acids in the liver and fat cells.

The Pentose Phosphate Pathway The pentose phosphate pathway begins with glucose-6-phosphate from reaction 1 in glycolysis, which is converted to ribulose-5-phosphate and requires two NADP+. Ribulose-5-phosphate is isomerized by the catalyst phosphopentose isomerase to ribose-5-phosphate, an important component of nucleotides such as ATP, GTP, UTP, NAD+, FAD, and RNA.

The Pentose Phosphate Pathway In a series of reactions, three ribose-5-phosphate molecules are converted to two hexose molecules and one triose molecule.

The Pentose Phosphate Pathway The pentose phosphate pathway converts glucose-6-P to ribose-5-P, which is needed for the synthesis of DNA and RNA. Ribose-5-P can also return to glycolysis as needed.