Biochemistry and Human Nutrition

Module 3. Metabolism

Lesson 16
GLYCOLYSIS

16.1 Introduction

• Glycolysis is a set of reactions that take place in cytoplasm of prokaryotes and eukaryotes.
• Glycolysis is an almost universal central pathway of glucose catabolism .
• The major roles of glycolysis are to produce energy and to produce intermediates for biosynthetic pathways.

16.2 Glycolysis has two Phases

• Preparatory phase
• Pay-off Phase

• In the preparatory phase of glycolysis, two molecules of ATP are invested and the hexose chain is cleaved into two triose phosphates.
• The payoff phase of glycolysis includes the energy-conserving phosphorylation steps in which some of the free energy of the glucose molecule is conserved in the form of ATP.
• Remember that one molecule of glucose yields two molecules of glyceraldehyde 3-phosphate; both halves of the glucose molecule follow the same pathway in the second phase of glycolysis. The conversion of two molecules of glyceraldehyde 3-phosphate to two molecules of pyruvate is accompanied by the formation of four molecules of ATP from ADP.
• However, the net yield of ATP per molecule of glucose degraded is only two, because two ATP were invested in the preparatory phase of glycolysis to phosphorylate the two ends of the hexose molecule.

• For each molecule of glucose degraded to pyruvate, two molecules of ATP are generated from ADP and Pi.

and the formation of ATP from ADP and Pi, which is endergonic:

However, complete oxidation of glucose to carbon dioxide and water proceeds with a standard free-energy change of -2,840 kJ/mol.

16.3 Irreversible / Regulatory steps in glycolysis

16.3.1 Hexokinase

Hexokinase is present in all cells of all organisms. Hepatocytes also contain a form of hexokinase called hexokinase IV or glucokinase, which differs from other forms of hexokinase in kinetic and regulatory properties. Two enzymes that catalyze the same reaction but are encoded in different genes are called isozymes.

16.3.2 Phosphofructokinase

The PFK-1 reaction is essentially irreversible under cellular conditions, and it is the first “committed” step in the glycolytic pathway; glucose 6-phosphate and fructose 6-phosphate have other possible fates, but fructose 1,6-bisphosphate is targeted for glycolysis. Phosphofructokinase-1 is a regulatory enzyme. The activity of PFK-1 is increased whenever the cell’s ATP supply is depleted or when the ATP breakdown products, ADP and AMP (particularly the latter), are in excess. The enzyme is inhibited whenever the cell has ample ATP and is well supplied by other fuels such as fatty acids. (Fig. 16.1 Glycolysis)

16.3.3 Pyruvate kinase

The last step in glycolysis is the transfer of the phosphoryl group from phosphoenolpyruvate to ADP, catalyzed by pyruvate kinase, which requires K+and either Mg ²⁺ or Mn ²⁺

16.4 Pasteur Effect

Louis Pasteur discovered that both the rate and the total amount of glucose consumption were many times greater under anaerobic than aerobic conditions. The ATP yield from glycolysis under anaerobic conditions (2 ATP per molecule of glucose) is much smaller than that from the complete oxidation of glucose to CO₂ under aerobic conditions (30 or 32 ATP per glucose). About 15 times as much glucose must therefore be consumed anaerobically as aerobically to yield the same amount of ATP.

16.5 Substrate-level phosphorylation

The enzyme phosphoglycerate kinase transfers the high-energy phosphoryl group from the carboxyl group of 1,3-bisphosphoglycerate to ADP, forming ATP and 3- phosphoglycerate. Thus by consuming the product of 1,3-bisphosphoglycerate of previous step, keeps [1,3-bisphosphoglycerate] relatively low in the steady state. The outcome of these coupled reactions, both reversible under cellular conditions, is that the energy released on oxidation of an aldehyde to a carboxylate group is conserved by the coupled formation of ATP from ADP and Pi. The formation of ATP by phosphoryl group transfer from a substrate such as 1,3-bisphosphoglycerate is referred to as a substrate-level phosphorylation, to distinguish this mechanism from respiration-linked phosphorylation. Substrate-level phosphorylations involve soluble enzymes and chemical intermediates (1,3-bisphosphoglycerate in this case). Respiration-linked phosphorylations, on the other hand, involve membrane-bound enzymes and transmembrane gradients of protons

In the overall glycolytic process, one molecule of glucose is converted to two molecules of pyruvate (the pathway of carbon). Two molecules of ADP and two of Pi are converted to two molecules of ATP (the pathway of phosphoryl groups). Four electrons, as two hydride ions, are transferred from two molecules of glyceraldehydes 3-phosphate to two of NAD+ (the pathway of electrons).

16.6 Fate of Pyruvate

16.6.1 Entry into the citric acid cycle

Glycolysis releases relatively little of the energy present in a glucose molecule; much more is released by the subsequent operation of citric acid cycle and oxidative phosphorylation. Under aerobic conditions, pyruvate is converted to acetyl Co-A by the enzyme pyruvate dehydrogenase which enters the citric acid cycle. (Fig. 16.2)

16.6.2 Conversion to fatty acids or ketone bodies

When the cellular energy level is high (ATP in excess), the rate of citric acid cycle decreases and acetyl Co-A begins to accumulate and is used for fatty acid or ketone body synthesis

16.6.3 Conversion to lactate

The NAD+ used during glycolysis in the formation of 1,3 biphosphoglycerate by glyceraldehyde 3- phosphate dehydrogenase must be regenerated if glycolysis has to continue. Under aerobic conditions NAD+ is regenerated by reoxidation of NADH via electron transport chain. However, when oxygen is limiting as in muscle during exercise reoxidation of NADH to NAD+ by ETC is insufficient to maintain glycolysis . Hence NAD+ is regenerated by conversion of the puruvate to lactate by lactate dehydrogenase. (Fig. 16.3 )

16.6.4 Alcoholic fermentation

In microbes, NAD+ is required for continuation of glycolysis under anaerobic conditions. So, pyruvate is converted to acetaldehyde by pyruvate decarboxylase and then to ethanol by alcohol dehydrogenase. The last reaction simultaneously reoxidizes the NADH to NAD. (Fig. 16.4)

16.7 Entry of other Carbohydrates in Glycolysis

Many carbohydrates besides glucose meet their catabolic fate in glycolysis, after being transformed into one of the glycolytic intermediates. The most significant are the storage polysaccharides glycogen and starch; the disaccharides maltose, lactose, trehalose, and sucrose; and the monosaccharides fructose, mannose, and galactose .

Glycogen in animal tissues can be mobilized for use within the same cell by a phosphorolytic reaction catalyzed by glycogen phosphorylase. This enzyme catalyzes an attack by Pi on the (α1-4) glycosidic linkage that joins the last two glucose residues at a nonreducing end, generating glucose 1-phosphate and a polymer one glucose unit shorter. A debranching enzyme removes the branches α1-6 glucosidic linkage. (Fig. 16.5)