Digestion of carbohydrates

Carbohydrates of foods are carbohydrate source of the organism. Usage rate is 400-500 grams per day. Carbohydrates are the major source of energy needed to man. Carbohydrates as a part of glycoproteins also performe protective function. They are used for synthesis of NA.

Starch is accumulated in plants, lactose is present in breast milk, glucose and fructose are in honey and fruits, and maltose comes from the foods in which starch is partially hydrolyzed, such as malt.

Annual consumption of carbohydrates is 400-500 g/day, with them comes the principal amount of calories needed to man.

Food poly- and disaccharides undergo enzymatic digestion in the digestive tract, where the enzymatic hydrolysis of glycosidic bonds goes. Monosaccharides are formed, then they are absorbed, enter the blood and tissues.

a-amylase of saliva decomposes a-1, 4-glycosidic bonds. Starch is partially digested in the mouth. Dextrins and maltose are formed.

Gastric juice does not contain enzymes that break down dietary carbohydrates. Saliva amylase is inactivated in the stomach, because pH of gastric juice is 2, and pH optimum of amylase of saliva is 6.7. But amylase acts any time inside the food bolus.

Pancreatic amylase hydrolyzes starch in the upper small intestine by sequential cleavage of disaccharide residues. Maltose and isomaltose are formed.

Maltose, isomaltose, sucrose, lactose are hydrolyzed by glycosidases on the cell surface of the small intestine to monomers (maltose is hydrolyzed by the enzyme maltase to glucose, sucrose is hydrolyzed by the enzyme sucrase to glucose and fructose).

b-Cellulose is not cleaved in the gastrointestinal tract: a person cannot produce an enzyme hydrolyzing 1, 4-glycoside bonds. Undigested cellulose of plant foods promotes normal bowel movements.

Glucose transport across the membrane occurs by facilitated diffusion or active transport.

Na⁺-glucose co-transporter, or simporter, carries the secondary active transport of glucose. Concentration gradient of Na⁺ is maintained through the work of Na⁺, K⁺-pump. Galactose is transported by the same way.

Glucose can be transported across the membrane also by facilitated diffusion with the help of carrier proteins. There are five types of glucose transporters (GLUT).The rate of the transmembrane transport of glucose depends on its concentration gradient. Exceptions are muscle cells and adipose tissue, where there are insulin-dependent transporters. In the absence of insulin, the membrane is impermeable to glucose.

Fructose is also absorbed by facilitated diffusion.

Hereditary or acquired defects of enzymes that hydrolyze carbohydrates is one of the causes of digestive disorders. The accumulation of undigested carbohydrates increases the flow of water into the lumen of the intestine that causes spasms and diarrhea. The action of bacteria on non-hydrolyzed carbohydrates leads to flatulence.

Most of the glucose (90%) comes from the blood through the portal vein to the liver. In the cell, glucose is phosphorylated (the active form) and subjected to further transformations.

 

GLYCOGEN METABOLISM

Glycogen is the main reserve homopolysaccharide of a human. A monomer of glycogen is glucose. Glucose residues are connected in linear chains by 1, 4-a-glycoside bonds. Branches are formed by1, 6-a-glycoside bonds. The branched structure of glycogen causes many terminal monomers. It is important for enzymes working at glycogen decay or synthesis. Glycogen is deposited in the liver and skeletal muscle and stored in the cytosol in the form of pellets. Metabolic pathways of glycogen synthesis and degradation are different.

Glycogen synthesis (glycogenesis) occurs within 1-2 hours after intake of carbohydrate foods and requires consumption of ATP. This process reduces the glucose level in the blood.

1). Phosphorylation of glucose with the participation of hexokinase (in muscle) and glucokinase (in liver) gives glucose-6-phosphate, which transforms into glucose-1-phosphate (enzyme is phosphoglukomutase):

2). Glucose-1-phosphate reacts with UTP with the participation of glucose-1-phosphate uridil transferase. UDP-glucose and pyrophosphate are formed:

3). Transfer of glucose residue from UDP-glucose to a small fragment of pre-existing glycogen chain (“primer”) goes under the action of glycogen synthase:

UDP-G + glycogen (n) ® UDP + glycogen (n+ 1)

Glycogen synthase catalyzes the formation of a-1, 4-glycosidic bonds. In the absence of glycogen a specific protein glycogenin can accept glucose from UDP-glucose.

Glycogen branching enzyme (glucosyl α -4, 6 transferase) provides formation of 1, 6-α glycosidic bonds. It transfers the oligosaccharide fragments (6-7 glucose residues) to the 6-hydroxyl group of glucose residue of the same or different chains of glycogen where it is linked by α -1, 6-bond.

The degradation of glycogen (glycogenolysis) is a conversion of glycogen from reserve form in the metabolic form (glucose). In the presence of phosphorylase glycogen is degraded into glucose-1-phosphate without splitting on fragments.

Glycogen phosphorylase exists in two forms - phosphorylase a (active) and phosphorylase b (inactive). Both forms can dissociate into subunits. Phosphorylase b consists of two subunits, phosphorylase a consists of 4 subunits. The conversion of phosphorylase a into phosphorylase b is accomplished by phosphorylation of protein under the action of phosphorylase kinase:

2 Phosphorylase b + 4 ATP → phosphorylase a + 4 ADP.

Inactive phosphorylase kinase becomes active under the influence of the enzyme cAMP-dependent protein kinase. cAMP is formed from ATP under the action of adenylate cyclase that is activated by adrenalin and glucagon. As a result, glycogen breaks down and forms glucose-1-phosphate.

A limit dextrin is formed, which is cleaved under the action of a bifunctional enzyme (debranching enzyme). It has transferase activity and α -1, 6-glucosidase activity and releases a free glucose. Then glycogen again can be degraded by phosphorylase.

Glucose-1-phosphate under the influence of phosphoglucomutase is converted into glucose 6-phosphate. Glucose is formed in the liver from glucose-6-phosphate by hydrolytic cleavage of phosphate (enzyme is glucose-6-phosphatase).

G-1-P ® G-6-P ® G

 

GLYCOLYSIS

Glycolysis (Embden – Meyerhof pathway) is the sequence of enzymatic reactions that lead to the splitting of glucose with the formation of pyruvic acid or lactate, accompanied by the formation of ATP. This process occurs in the cell cytosol.

In aerobic glycolysis pyruvate is formed. It enters the mitochondria, and further, in the common catabolic pathway, is oxidized to CO₂ and H₂O. Aerobic glycolysis is the part of aerobic degradation of glucose.

In anaerobic glycolysis pyruvate is formed and then is converted to lactate. Anaerobic degradation of glucose and anaerobic glycolysis are synonyms. Anaerobic glycolysis occurs in the first few minutes of muscular work, in erythrocytes (there are no mitochondria in erythrocytes), and when there is insufficient intake of oxygen.

Reactions of glycolysis:

1). Phosphorylation of glucose. The reaction is catalyzed by hexokinase, in parenchymal liver cells it is catalyzed by glucokinase. The formation of glucose-6-phosphate in the cell is a trap for glucose, because membrane is impermeable for phosphorylated glucose. It is irreversible reaction. Hexokinase has low K_m, glucokinase has high K_m.

2). Isomerization reaction is catalyzed by glucose-6-phosphate isomerase (phosphoglucose isomerase):

3) Phosphorylation reaction is catalyzed by 6-phosphofructokinase. This is irreversible and the rate-limiting reaction.

These three reactions are reactions of energy investment phase.

4). Aldol splitting reaction is catalyzed by aldolase.

5). Isomerization of dihydroxyacetonephosphate is catalyzed by enzyme triosephosphate isomerase:

1 molecule of glucose is converted to 2 molecules of glyceraldehyde-3-phosphate (reactions 4, 5). This was splitting phase.

The next stage is energy generation phase.

6). The oxidation of glyceraldehyde-3-phosphate goes under the action of enzyme glyceraldehyde-3-phosphate dehydrogenase:

7). Substrate level phosphorylation goes on with the participation of phosphoglycerate kinase:

8). Intramolecular transfer of a phosphate group. The enzyme is phosphoglycerate mutase:

9). Dehydration with the participation of enolase:

10). Substrate level phosphorylation. The enzyme is pyruvate kinase:

This is irrevesible reaction.

11). Under anaerobic conditions, the reduction of pyruvate to lactate occurs. Reaction is catalyzed by lactate dehydrogenase:

The overall equation of anaerobic glycolysis:

glucose + 2 NAD⁺ + 2 ADP + 2 P_i ® 2 pyruvate + 2 NADH + 2 ATP

Anaerobic glycolysis does not require the mitochondrial respiratory chain.

The yield of ATP from anaerobic glycolysis: ATP is formed in two reactions of substrate level phosphorylation. 4 ATP molecules are formed by 1 glucose molecule (1 glucose molecule gives 2 trioses, and each gives 2 ATP in 7^th and 10^th reactions of glycolysis.). 2 ATP are consumed (reaction 1 and 3 of glycolysis) and

4 ATP - 2 ATP = 2 ATP.

The yield of ATP in aerobic degradation of glucose:

- three reactions of substrate level phosphorylation (7^th, 10^th - glycolysis; 5^th – Krebs cycle) = 3ATP;

- five reactions of dehydrogenation with the formation of NADH (6^th - glycolysis, 3^rd - oxidative decarboxylation of pyruvate; 3^rd, 4^th, 8^th - Krebs cycle) Þ 3x5 = 15ATP;

- dehydrogenation reaction (6^th - Krebs cycle) with the formation of FADH₂ = 2 ATP;

total: 3ATP + 15 ATP + 2ATP= 20 ATP.

From 1 molecule of glucose 2 molecules of glyceraldehydes-3-phosphate are produced, then 2 х 20ATP = 40 ATP.

In reactions 1 and 3 of glycolysis 2ATP are consumed, and

40ATP - 2ATP = 38ATP.

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