Digestion of Carbohydrates Glycolysis: Oxidation of Glucose Pathways for Pyruvate

Metabolic Pathways 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

Dig

est

ion

of

Carb

oh

yd

rate

s

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

55

Glycolysis: Energy-Production In reactions 7 and 10, the hydrolysis of phosphates

in the triose phosphates generates four ATP molecules.

Glycolysis: Reactions 6-10

6

7

8

9

10

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: Overall Reaction

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.

Glycolysis

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.

When oxygen is present in the cell, (aerobic conditions), pyruvate from glycolysis is decarboxylated to produce acetyl CoA and CO2 by enzyme called Pyruvate Dehydrogenase

Pathways for Pyruvate

O || CH3—C—COOH + HS—CoA + NAD+

pyruvic acid

O||

CH3—C—S—CoA + CO2 + NADH + H+

acetyl CoA

When oxygen is not available (anaerobic conditions), pyruvate is reduced to lactate, which replenishes NAD+ to continue glycolysis.

Lactate Formation

O ||

CH3—C—COO- + NADH + H+

pyruvate

OH |

CH3—CH—COO- + NAD+

lactate

Lactate dehydrogenase

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

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