Carbohydrates metabolism. Digestion and absorption of carbohydrates. Intermediate metabolism of carbohydrates

The role of carbohydrates

The ration of proteins, lipids and carbohydrates in food should be 1:1:4, i.e. must be supplied through the diet 400-500g of carbohydrates or 124g per 1000kcal a day.it is desirable that disaccharides made up 25% from this quantity. Overeating of carbohydrates results in obesity and atherosclerosis.

The role of carbohydrates:

1. Energetical function: 99% of carbohydrates is used for energy production. The oxidation of 1g of glucose gives 17,2kj (4,1kcal) of energy;

2. Structural role: formation of GAG, glycoproteins, and other compounds (nucleic acids, nucleoproteins, glycolipids, nucleosidedi and triphosphates, coenzymes, glucuronic acid);

3. reserve function: glycogen is a storage of glucose

4. signal role: receptors and some hormones contain carbohydrates. Carbohydrates stipulate antigenicity of tissues

5. food fibers promotes the retention of water in intestine and formation of feces.

The diet riched by food fibers decreases possibility of appearance of diverticulitis, cancer of large intestine, heart-vessel diseases and diabetes mellitus. Cellulose favourably acts on function of large intestine. Pitch and pectins decrease the level of cholesterol in blood. Perhaps it is due to the binding of bile acids and cholesterol. Pitch and pectins stop evacuation od acydchym from gaster, decrease level of glucose in blood.

The digestion of carbohydrates

The digestion of the starch and glycogen of food beginns in mouth under action of salivary amylase. Gaster doesn’t contain glycosidases, but food undergoes the cleavage under influence of salivary amylase. The action of amylase continues in small intestine. Some other enzymes take part in digestion of starch and glycogen of food, e.g. pancreatic amylase:

(C6H10O5)n + (H2O)n  (Maltose)n

maltose is broken up to 2 glucoses in small intestine under action of maltase (intestinal enzyme)

Lactose is broken up to beta-galactose and glucose in small intestine under action of lactase (intestinal enzyme)

Sucrose is hydrolyzed under action of intestinal sucrase to fructose and glucose:

The digestion of nucleic acids gives pentoses

Table 7 The absorption of monosaccharides

Lumen of intestine	Wall of intestine	Portal vein
Galactose Glucose Fructose Pentoses	Na-Glucose transporter (act. tr.) Galactose Glucose GLUT (facilitated diffusion) Fructose ----------simple diffusion	GLUT (Facilitated diffusion) Galactose Glucose Fructose Pentoses All monosaccharides ↓ liver ↓ glucose ↓ blood circulation

The conversion of glucose in tissues. Glucostatic function of liver

The following tissues and organs use glucose particularly intensive:

1. nervous tissue because glucose is only source of energy

2. muscles (for production of energy fro contraction)

3. wall of intestine (absorption demands the energy)

4. kidneys (formation of urine is energy dependent process)

5. adrenal glands (energy is necessary for synthesis of hormones)

6. erythrocytes

7. adipose tissue (glucose is necessary as source of glycerol)

8. mammary glands, particularly in lactation (glucose is necessary for lactose formation)

65% of glucose is oxidized; 30% of glucose is used for liponeogenesis; 5% of glucose is used on glycogen formation.

Glucostatic function of liver: is provided by 3 processes:

1. glycogenogenesis (formation of glycogen)

2. glycogenolysis (cleavage of glycogen)

3. gluconeogenesis (formation of glucose from intermediates).

Increase of glucose in blood causes increase of glycogenogenesis. Decrease of glucose in blood causes glycogenolysis and gluconeogenesis.

The normal level of glucose in blood is 3.4-6.1mM/L and liver regulates concentration of glucose in blood

Glycogenogenesis 2/5 of glycogen is stored in liver. 1/5 of glycogen is formed in muscles and other organs and tissues. Glycogen is a reserve of glucose. It is a high molecular compound and therefore doesn’t increase osmotic pressure in cells. This process is complex and consists of following stages:

ATP ADP UTP PP UDP

Glucose------------ Glucose-6-ph---------UDP-glucose -------- (C6H10O5)n+1

Glycogenolysis 1) Phosphorolysis; 2) hydrolysis

Phosphorolysis occurs in different tissues:

Phosphorylase

(C6H10O5)n ---------------------- (C6H10O5)n-1 + Gl-1-Ph

+H3PO4 ↓ Gl-6-ph-ase

oxidation←Gl-6-Ph----------glucose (in liver)

Hydrolysis of glycogen occurs in liver only:

γ-amylase

(C6H10O5)n ----------------------- (C6H10O5)n-1 + Glucose

+HOH

Anaerobic glycolysis and its energetic value

1) Glucose+ATPglucose-6-phosphate+ADP (glucokinase)

2) glucose-6-phosphate↔fructose-6-phosphate

3) fructose-6-phosphate+ATP fructose-1,6-diphosphate+ADP (fructokinase)

4) fructose-1,6-diphosphate↔phosphoglyceraldehyde(PGA)+ dioxyacetone phosphate (DOAP) (aldolase)

5) 2PGA+2NAD+2H3PO4↔2(1,3diphosphoglycerate)

6) 2(1,3diphosphoglycerate)+2ADP↔2ATP+2(3-phosphoglycerate) (substrate phosphorylation)

7) 2(3-phosphoglycerate) ↔ 2(2-phosphoglycerate)

8) 2(2-phosphoglycerate) ↔ 2(phosphoenolpyruvate)+2H2O (enolase)

9) 2(phosphoenolpyruvate) +2ADP2(pyruvic acid)+2ATP (pyruvate kinase)

10) 2(pyruvic acid) +2NADH2 ↔ 2 lactic acid +2NAD (lactate dehydrogenase)

Energetic value of anaerobic glycolysis:

If glycolysis beginns from glucose 2 ATP is used for formation of glucose-6-phosphate and fructose-1,6-diphosphate. Glycolysis gives 4 ATP, hence 2 ATP is remained. If glycolysis beginns from gl-6-Ph formed in glycogenolysis 1 ATP is used for formation of fructose-1,6-diphosphate, hence 3ATP is remained.

The importance of glycolysis:

1. glycolysis is a source of energy in some organs and tissues, e.g. in erythrocytes, lens, retina

2. this process is emergency pathway because it provides by energy in hypoxia and anoxia, e.g. in mountains, intoxication, anemia, diseases of heart, lungs, and vessels, hard work.

3. Some intermediates of glycolysis are used for synthetic processes, e.g. phosphotrioses, pyruvic acid, lactic acid cab be used on formation of glucose (gluconeogenesis), liponeogenesis and synthesis of unessential amino acids