Post on 19-Dec-2015
1
23.4 Digestion of Carbohydrates
23.5 Glycolysis: Oxidation of Glucose
23.6 Pathways for Pyruvate
Chapter 23 Metabolic Pathways for Carbohydrates
2
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.
Stage 1: Digestion of Carbohydrates
4
Glycolysis In Stage 2, the
metabolic pathway called glycolysis degrades glucose (6C) obtained from digestion to pyruvate (3C).
5
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.
7
Glycolysis: Energy-Production
In reactions 7 and 10, the hydrolysis of phosphates in the triose phosphates generates four ATP molecules.
9
Glycolysis generates 2 ATP and 2 NADH. Two ATP are used in energy-investment to add
phosphate groups to glucose and fructose-6-phosphate.
Four ATP are formed in energy-generation by direct transfers of phosphate groups to four ADP.
Glucose + 2ADP + 2Pi + 2NAD+ 2Pyruvate + 2ATP + 2NADH + 2H+
Glycolysis: Overall Reaction
10
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.
11
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 O
|| CH3—C—S—CoA + CO2 + NADH + H+
acetyl CoA
Pathways for Pyruvate
12
When oxygen is not available (anaerobic conditions), pyruvate is reduced to lactate, which replenishes NAD+ to continue glycolysis.
O lactate || dehydrogenase
CH3—C—COO- + NADH + H+
pyruvate OH |
CH3—CH—COO- + NAD+
lactate
Lactate Formation
13
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.
Lactate in Muscles
14
Fermentation
Fermentation: Occurs in anaerobic microorganisms such as yeast. Decarboxylates pyruvate to acetaldehyde, which is
reduced to ethanol. Regenerates NAD+ to continue glycolysis. O OH
|| | CH3—C—COOH + NADH + H+ CH3—CH2 + NAD+ + CO2
Pyruvic acid Ethanol