DIGESTION OF CARBOHYDRATES

In the diet, carbohydrates are present as complex polysaccharides (starch, glycogen), and to a minor extent, as disaccharides (sucrose and lactose). Polysaccharides are hydrolysed to monosaccharide units in the gastrointestinal tract.

This process of digestion starts in mouth by the salivary alpha-amylase. However, the time available for digestion in the mouth is limited because the gastric hydrochloric acid will inhibit the action of salivary amylase.

In the pancreatic juice another alpha-amylase is available which will hydrolyse the glycosidic linkages randomly in small intestine, so as to produce smaller subunits like maltose, isomaltose and branched or unbranched oligosaccharides.

The cells of brush border of intestine contain the enzymes, sucrase, maltase, isomaltase and lactase which hydrolyse the corresponding disaccharides into monosaccharides which are then absorbed.

Clinical Application; Lactose Intolerance

Lactase hydrolyses lactose to glucose and galactose. Lactase is present in the brush border of enterocytes. Deficiency of lactase leads to lactose

intolerance. In this condition, lactose accumulates in the gut. Irritant diarrhea and flatulence are seen.

Stages of catabolism

ABSORPTION OF CARBOHYDRATES

Only monosaccharides are absorbed by the intestine.

Absorption of Glucose

Glucose has specific transporters, which are transmembrane proteins.

1. Co-transport from Lumen to Intestinal Cell

This process is mediated by Sodium Dependent Glucose Transporter-1 (SGluT-1) .A membrane bound carrier protein is involved, which carries glucose, along with sodium. This sodium is later expelled by the sodium pump with utilization of energy. So energy is needed indirectly.

Clinical application: Common treatment for diarrhea is oral rehydration fluid. It contains glucose and sodium. Presence of glucose in oral rehydration fluid allows uptake of sodium to replenish body sodium chloride.

2. Another Uniport System Releases Glucose into Blood

The same intestinal epithelial cells have a different transport mechanism on the membrane facing capillaries. Intestinal cells release glucose into blood stream by the carrier mechanism called Glucose Transporter Type 2 (GluT2).This transporter is not dependent on sodium. It is a uniport, facilitated diffusion system.

GluT2 (facilitated transport) is involved in absorption of glucose from blood stream to cells. GluT2 is present in intestinal epithelial cells, liver cells, beta cells of pancreas and kidney. Since GluT2 has a high Km for glucose, its presence in beta cells is ideally suited for sensing a high glucose level and releasing insulin. So this mechanism enables the pancreas to monitor the glucose level and adjust the rate of insulin secretion.

3. Glucose Transporter 4

GluT4 is the major glucose transporter in skeletal muscle and adipose tissue. GluT4 is under the control of insulin. But other glucose transporters are not under the control of insulin.

Clinical application: Insulin induces the movement of intracellular GluT4 molecules to the cell surface and thus increases glucose uptake. In Type 2 diabetes mellitus , membrane GluT4 is reduced, leading to insulin resistance in muscle and fat cells. In diabetes, entry of glucose into muscle is only half of normal cells. Glucose is the preferred source of energy for most of the body tissues. Brain cells derive the energy mainly from glucose. When glucose metabolism is deranged, life threatening conditions may occur. Normal fasting plasma glucose level is 70 to 110 mg/dl. After a heavy carbohydrate meal, it rises; but in a normal person, this level is below 150 mg/dl.

Fate of absorbed sugars:

Monosaccharides (glucose, galactose and fructose) resulting from carbohydrate digestion are absorbed and undergo the following:

A. Uptake by tissues (liver):

After absorption the liver takes up sugars, where galactose and fructose are converted into glucose.

B. Glucose utilization by tissues:

Glucose may undergo one of the following fates:

1. Oxidation: through

a) Major pathways (glycolysis and Krebs' cycle) for production of energy.

b) Hexose monophosphate pathway: for production of ribose, deoxyribose and NADPH + H+

c) Uronic acid pathway, for production of glucuronic acid, which is used in detoxication and enters in the formation of mucopolysaccharide.

2. Storage: in the form of:

a) Glycogen: glycogenesis.

b) Fat: lipogenesis.

3. Conversion: to substances of biological importance:

a) Ribose, deoxyribose RNA and DNA.

b) Lactose milk.

c) Glucosamine, galactosamine mucopolysaccharides.

d) Glucoronic acid mucopolysaccharides.

e) Fructose in semen.

Aerobic respiration - the process by which a cell uses O2 to "burn" molecules and release energy

C6H12O6 + 6O2 >> 6CO2 + 6H2O

This reaction takes place over the course of three major reaction pathways:

1. Glycolysis

2. The Krebs Cycle

3. Electron Transport Phosphorylation

GLYCOLYSIS

In the pathway of glycolysis, glucose is split into two 3-carbon pyruvate molecules under aerobic conditions; or lactate under anaerobic conditions, along with production of a small quantity of energy.

(Or glucokinase)

Summary of glycolysis (Embden-Meyerhof pathway)

The whole reaction is summarized as

Glucose + 2 Pi + 2 ADP --> 2 Lactate + 2 ATP

· Notes

· Steps 1, 3 and 9 are key enzymes; these reactions are irreversible. Steps 5, 6 and 9 produce energy. Steps 5 and 10 are coupled for regeneration of NAD+.

· The steps 1,2 and 3 together are called as the preparatory phase, The steps 4 and 4-A are together called the splitting phase.

· Hexokinase and glucokinase may be considered as iso-enzymes; their properties are compared in Table below. Glucokinase is under the influence of insulin; but hexokinase is not.

· The phosphorylation of glucose to glucose-6-phosphate traps it within the cells and has to be metabolized.

· The enzyme phosphofructokinase (PFK) is an allosteric, inducible, regulatory enzyme. It is an important key enzyme of this pathway. This is an activation process, the energy being derived by hydrolysis of ATP. This irreversible step is the rate limiting reaction in glycolysis.

· The energy of bisphospho glycerate (1,3-BPG) is trapped to synthesize one ATP molecule with the help of bisphospho glycerate kinase. This is an example of substrate level phosphorylation, where energy is trapped directly from the substrate, without the help of the complicated electron transport chain reactions. When energy is trapped by oxidation of reducing equivalents such as NADH, it is called oxidative phosphorylation.

· In the 5th step, for each molecule of glucose entering in the pathway, two molecules of NAD+ are reduced to NADH. The availability of co-enzymes inside a cell is limited. Therefore, this step becomes a bottleneck in the whole reaction sequence.

For smooth operation of the pathway, the NADH is to be reconverted to NAD+. This can be done by oxidative phosphorylation. However, during

exercise, there is lack of oxygen. So this reconversion is not possible. Therefore, the cell has to couple some other reaction in which NAD+ is

regenerated in the cytoplasm itself. Hence, pyruvate is reduced to lactate; the NAD+ thus generated is reutilized for uninterrupted operation of the pathway. But when oxygen is in plenty, the two NADH molecules, generated in the glyceraldehyde- 3-phosphate dehydrogenase reaction (step 5), can enter the mitochondrial electron transport chain for complete oxidation. As each NADH provides 3 ATPs.

In RBCs, there are no mitochondria (where oxidative phosphorylation occurs). Hence RBCs derive energy only through anaerobic glycolysis, where the end product is lactic acid.

· Enolase (step 8) requires Mg++, fluoride irreversibly inhibit this enzyme by removing magnesium ions. Thus, fluoride will stop the whole glycolysis. So when taking blood for sugar estimation, fluoride is added to blood. If not, glucose is metabolized by the blood cells, so that lower blood glucose values are obtained.

Factors Regulating Glycolysis

A. Glucokinase that is active mainly in liver has a high Km for glucose and low affinity. Hence, glucokinase can act only when there is adequate glucose supply so that excess can be stored. Hexokinase with low km and high affinity can phosphorylate glucose even at lower concentrations so that glucose is made available to brain, cardiac and skeletal muscle.

Glucokinase can act only when there is plenty of glucose. Thus, when the supply of glucose is limited, glucose is made available to brain and muscles. Insulin increases its activity where as glucagon inhibits.

B. Pyruvate Kinase catalyses an irreversible step and is a regulatory enzyme of glycolysis. When energy is plenty in the cell, glycolysis is inhibited; Pyruvate kinase is inactive in the phosphorylated state.

Insulin favors glycolysis by activating the above two key glycolytic enzymes.

Glucagon and glucocorticoids inhibit glycolysis and favor gluconeogenesis.

Regulatory enzymes of glycolysis

Significance of the Glycolysis Pathway

1. It is the only pathway that is taking place in all the cells (cytoplasm) of the body.

2. Glycolysis is the only source of energy in erythrocytes.

3. In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis forms the major source of energy for muscles.

4. The glycolytic pathway may be considered as the preliminary step before complete oxidation.

5. The glycolytic pathway provides carbon skeletons for synthesis of non-essential amino acids as well as glycerol part of fat (glycerol is required which can be derived from glucose through DHAP also glycerol portion of the neutral fat can enter into glycolytic or gluconeogenic pathways at step 4).

6. Most of the reactions of the glycolytic pathway are reversible, which are also used for gluconeogenesis.

Clinical Applications of Glycolytic Enzymes

1. Lactic acidosis may be seen in hypoxia, shock, pulmonary failure, alcohol abuse and diabetes mellitus .

2. Deficiency of glycolytic enzymes. These conditions are rare, out of which pyruvate kinase deficiency and hexokinase deficiency are comparatively common. Though rare, these deficiency states can lead to hemolytic anemia, because energy depleted RBCs are destroyed. Hexokinase deficient RBCs have a low level of 2,3 BPG and a high affinity for oxygen. On the other hand, a deficiency of Pyruvate kinase leads to decreased affinity for oxygen since 2,3 BPG levels are high. Inherited aldolase deficiency also causes hemolysis. In PFK deficiency, muscle weakness is seen.

ALTERNATE FATES OF PYRUVATE

A. Oxidative decarboxylation of pyruvate

Oxidative decarboxylation of pyruvate by pyruvate dehydrogenase complex is an important pathway in tissues with a high oxidative capacity, such as cardiac muscle. Pyruvate dehydrogenase irreversibly converts pyruvate, the end product of glycolysis, into acetyl CoA, a major fuel for the tricarboxylic acid cycle.

B. Carboxylation of pyruvate to oxaloacetate

Carboxylation of pyruvate to oxaloacetate (OAA) by Pyruvate carboxylase is a biotin-dependent reaction. This reaction is important because it replenishes the citric acid cycle intermediates, and provides substrate for gluconeogenesis.

C. Reduction of pyruvate to lactate

Reduction of pyruvate to lactate by lactate dehydrogenase under anaerobic condition