Metabolic Pathways of Carbohydrates Digestion of Carbohydrates Glycolysis: Oxidation of Glucose Pathways for Pyruvate

Digestion of Carbohydrates In the mouth, salivary amylase hydrolyzes -glycosidic bonds in polysaccharides to give smaller polysaccharides (dextrins), maltose, and some glucose. In the small intestine, pancreatic amylase hydrolyzes dextrins to maltose and glucose. The disaccharides maltose, lactose, and sucrose are hydrolyzed to monosaccharides. The monosaccharides enter the bloodstream for transport to the cells.

Digestion of Carbohydrates

Glycolysis In Stage 2, the metabolic pathway called glycolysis degrades glucose (6C) obtained from digestion to pyruvate (3C).

Glycolysis: Energy-Investment In reactions 1-5 of glycolysis: Energy is used to add phosphate groups to glucose and fructose. Glucose is converted to two three-carbon molecules.

Glycolysis: Energy-Investment 1 2 3 4 5

Glycolysis: Energy-Production In reactions 7 and 10, the hydrolysis of phosphates in the triose phosphates generates four ATP molecules.

Glycolysis: Reactions 6-10 7 8 9 10

Glycolysis: Overall Reaction Glycolysis generates 2 ATP and 2 NADH. Two ATP are used in energy-investment to add phosphate groups to glucose and fructose-6-phosphate. 4 ATP are formed in energy-generation by direct transfers of phosphate groups to 4 ADP. Glucose + 2ATP + 4ADP + 4Pi + 2NAD+ 2Pyruvate + 4ATP + 2ADP + 2NADH + 2H+ Glucose + 2ADP + 2Pi + 2NAD+ 2Pyruvate + 2ATP + 2NADH + 2H+ Reoxidation of 2 NADH through Respiratory Chain will produce 6 ATP Total ATP gain = 4ATP -2ATP + 6ATP = 8 ATP

Glycolysis In this pathway glucose is converted to pyruvate or lactate, with production of energy (ATP). Glycolysis is the major oxidative pathway for the utilization of glucose and is found in all cells. It is unique pathway in that: It can utilize O2 (aerobic), in the cells with mitochondria to yield pyruvate. It can function in the absence of O2 (anaerobic) as in exercising muscle, or in cells that lack mitochondria (RBCs), to yield lactate. All enzymes of glycolysis are found in the cytosol.

Regulation of Glycolysis Reaction 1 Hexokinase is inhibited by high levels of glucose-6-phosphate, which prevents the phosphorylation of glucose. Reaction 3 Phosphofructokinase, an allosteric enzyme, is inhibited by high levels of ATP and activated by high levels of ADP and AMP. Reaction 10 Pyruvate kinase, another allosteric enzyme is inhibited by high levels of ATP or acetyl CoA.

Pathways for Pyruvate When oxygen is present in the cell, (aerobic conditions), pyruvate from glycolysis is decarboxylated to produce acetyl CoA and CO2 O || CH3—C—COOH + HS—CoA + NAD+ pyruvic acid CH3—C—S—CoA + CO2 + NADH + H+ acetyl CoA Pyruvate Kinase

Lactate dehydrogenase Lactate Formation When oxygen is not available (anaerobic conditions), pyruvate is reduced to lactate, which replenishes NAD+ to continue glycolysis. O || CH3—C—COO- + NADH + H+ pyruvate OH | CH3—CH—COO- + NAD+ lactate Lactate dehydrogenase

Lactate in Muscles Under anaerobic conditions (strenuous exercise): Oxygen in the muscles is depleted. Lactate accumulates in the muscles. Muscles tire and become painful. Rest is needed to repay the oxygen debt and to reform pyruvate in the liver.

Fermentation Occurs in anaerobic microorganisms such as yeast. Decarboxylates pyruvate to acetaldehyde, which is reduced to ethanol. Regenerates NAD+ to continue glycolysis. O || CH3—C—COOH + NADH + H+ Pyruvic acid OH | CH3—CH2 + NAD+ + CO2 Ethanol

Pathways for Pyruvate