GLYCOLYSIS

 GLYCOLYSIS

Glycolysis is derived from the Greek words (glycose—sweet or sugar; lysis—dissolution). It is a universal pathway in the living cells. The complete pathway of glycolysis was elucidated in 1940. This pathway is often referred to as the Embden-Meyerhof pathway (E.M. pathway) in honor of the two biochemists who made a major contribution to the knowledge of glycolysis. Glycolysis is defined as the sequence of reactions converting glucose (or glycogen) to pyruvate or lactate, with the production of ATP  In the pathway of glycolysis, glucose is split into two 3-carbon pyruvate molecules under aerobic conditions; or lactate under anaerobic conditions, along with the production of a small quantity of energy.

Salient features

1. Glycolysis takes place in all cells of the body. The enzymes of this pathway are present in the cytosolic fraction of the cell.

2. Glycolysis occurs in the absence of oxygen (anaerobic) or in the presence of oxygen (aerobic). Lactate is the end product under anaerobic conditions. In the aerobic condition, pyruvate is formed, which is then oxidized to CO2 and H2O.

3. Glycolysis is a major pathway for ATP synthesis in tissues lacking mitochondria, e.g. erythrocytes, cornea, lens, etc.

4. Glycolysis is very essential for the brain which is dependent on glucose for energy. The glucose in the brain has to undergo glycolysis before it is oxidized to CO2 and H2O.

5. Glycolysis (anaerobic) may be summarized by the net reaction Glucose + 2ADP + 2Pi o 2Lactate + 2ATP

6. Glycolysis is a central metabolic pathway with many of its intermediates providing branch points to other pathways. Thus, the intermediates of glycolysis are useful for the synthesis of amino acids and fat.

7. Reversal of glycolysis along with the alternate arrangements at the irreversible steps, will result in the synthesis of glucose (gluconeogenesis)

GLYCOLYSIS PATHWAY

Conversion of pyruvate to lactate—significance

Under anaerobic conditions (lack of O2), pyruvate is reduced by NADH to lactate in presence of the enzyme lactate dehydrogenase (competitive inhibitor—oxamate). The NADH utilized in this step is obtained from the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase. The formation of lactate allows the regeneration of NAD+ which can be reused by glyceraldehyde 3-phosphate dehydrogenase so that glycolysis proceeds even in the absence of oxygen to supply ATP. The occurrence of uninterrupted glycolysis is very essential in skeletal muscle during strenuous exercise where oxygen supply is very limited. Glycolysis in the erythrocytes leads to lactate production since mitochondria—the centers for aerobic oxidation—are absent. The brain, retina, skin, renal medulla, and gastrointestinal tract derive most of their energy from glycolysis.

Energy Yield from Glycolysis

I.                  During anaerobic (oxygen-deficient) conditions, when one molecule of glucose is converted to 2 molecules of lactate, there is a net yield of 2 molecules of ATP.

ii.              4 molecules of ATP are synthesized by the 2 substrate-level phosphorylations (steps 6 and 9). But 2 molecules of ATP are used in steps 1 and 3, hence the net yield is only 2 ATP.

iii.             The whole reaction is summarized as Glucose + 2 Pi + 2 ADP --> 2 Lactate + 2 ATP

iv.             But when oxygen is in plenty, the two NADH molecules, generated in the glyceraldehyde3-phosphate dehydrogenase reaction (step 5), can enter the mitochondrial electron transport chain for complete oxidation. As each NADH provides 2.5 ATPs, this reaction generates 2.5 x 2 = 5 ATPs. Thus when oxygen is available, the net gain of energy from the glycolysis pathway is 7 ATPs.

v.                Hence the ATP yield from glycolysis is different in anaerobic and aerobic conditions.

vi.             Pyruvate is later oxidatively decarboxylated to acetyl CoA (see below), which enters into the citric acid cycle. Complete oxidation of glucose through glycolysis plus citric acid cycle will yield a net of 32 ATPs

ANAEROBIC CONDITION

AEROBIC CONDITION

ENERGY YIELD OF GLYCOLYSIS

Glycolysis and shuttle pathways

In the presence of mitochondria and oxygen, the NADH produced in glycolysis can participate in the shuttle pathways for the synthesis of ATP. If the cytosolic NADH uses a malate-aspartate shuttle, 3 ATP are generated from each molecule of NADH. This is in contrast to the glycerolphosphate shuttle that produces only 2 ATP

Cancer and glycolysis

Cancer cells display increased uptake of glucose and glycolysis. As the tumors grow rapidly, the blood vessels are unable to supply adequate oxygen, and thus a condition of hypoxia exists. Due to this, anaerobic glycolysis predominantly occurs to supply energy. The cancer cells get adapted to hypoxicglycolysis through the involvement of a transcription factor namely hypoxia-inducible transcription factor (HIF). HIF increases the synthesis of glycolytic enzymes and glucose transporters. However, the cancer cells cannot grow and survive without proper vascularization. One of the modalities of cancer treatment is to use drugs that can inhibit the vascularization of tumors.

 Irreversible steps in glycolysis

Most of the reactions of glycolysis are reversible. However, the three steps catalyzed by the enzymes hexokinase (or glucokinase), phosphofructokinase and pyruvate kinase, are irreversible. These three stages mainly regulate glycolysis. The reversal of glycolysis, with alternate arrangements made at the three irreversible stages, leads to the synthesis of glucose from pyruvate (gluconeogenesis)

Regulation of Glycolysis  

The regulatory enzymes or key enzymes of glycolysis are:

1. Glucokinase/Hexokinase, step 1

2. Phosphofructokinase, step 3  

3. Pyruvate kinase, step 9

REGULATION OF ENZYMES IN GLYCOLYSIS

 Factors Regulating Glycolysis

 1. Glucokinase/Hexokinase: Phosphorylation of glucose by these enzymes is a reaction that is regulated by feedback inhibition (hexokinase by glucose-6-phosphate) and activated by insulin (glucokinase is induced by insulin). Glucokinase is active mainly in the liver has a high Km for glucose and low affinity. Hence, glucokinase can act only when there is an 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 the 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 the brain and muscles.

 2. Phosphofructokinase (PFK) (step 3) is the most important rate-limiting enzyme for the glycolysis pathway. ATP and citrate are the most important allosteric inhibitors. AMP acts as an allosteric activator.

 3. Fructose-2,6-bisphosphate (F-2,6-BP) increases the activity of phospho fructo kinase F-2,6-BP is formed from fructose-6-phosphate by the action of an enzyme called PFK-2. (It is different from the PFK1). Fructose-2,6- bisphosphate is hydrolyzed to fructose-6- phosphate by fructose-2,6-bisphosphatase. The activities of both the enzymes (PFK2 and fructose-2,6-bisphosphatase) are reciprocally regulated. When glucose supply is in plenty, PFK-2 is dephosphorylated and activated; so F2,6-BP concentration increases; this, in turn, activates PFK. Thus glycolysis is favored.

4. Pyruvate Kinase catalyzes an irreversible step and is a regulatory enzyme of glycolysis. When energy is plenty in the cell, glycolysis is inhibited Insulin increases its activity whereas glucagon inhibits. Pyruvate kinase is inactive in the phosphorylated state.

 5. Insulin favors glycolysis by activating the above three key glycolytic enzymes.

6. Glucagon and glucocorticoids inhibit glycolysis and favor gluconeogenesis

 Significance of the Glycolysis Pathway

1. It is the only pathway that is taking place in all the cells 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 the synthesis of non-essential amino acids as well as glycerol part of fat.

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