GLYCOLYSIS
The main aim of digestion of carbohydrates is to release their building block units, which are
catabolized, and the energy stored in the form of ATP molecules released for the performance
of work in tissues. Every tissue has its requirement for glucose. In some cases like in the brain,
the requirement is substantial while in others like erythrocytes, it is total.
Glycolysis is derived from the Greek glykys, “sweet” or “sugar,” and lysis, “splitting”. In the
process, a molecule of glucose is degraded in a series of enzyme catalyzed reactions to yield
two molecules of the three-carbon compound pyruvate. Glycolysis may be considered as the
pathway by which glucose is converted via fructose-1, 6-bisphosphate to pyruvate with a net
generation of 2 molecules of ATP per molecule of glucose. This sequence of ten enzymatic
reactions, which occur in the cytosol is probably the most completely understood biochemical
pathway. The pathway plays a key role in energy metabolism by providing a significant portion
of the energy utilized by most organisms. The glycolytic pathway is divided into two phases
(preparative phase and pay off phase). In the preparative phase, glucose is phosphorylated and
converted to glyceraldehydes-3-phosphate. The payoff phase is concerned with the oxidative
conversion of glyceraldehydes-3-phosphate to pyruvate and the coupled formation of ATP and
NADH.
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REACTIONS OF GLYCOLYSIS
Hexokinase
This is the first reaction of glycolysis (priming reaction) which involves the transfer of
phosphoryl group from ATP to glucose to form glucose-6-phosphate (G-6-P) in a reaction
catalyzed by hexokinase. A kinase is an enzyme that transfers phosphoryl groups between ATP
and a metabolite. The different body tissues possess different isoenzymes of hexokinase. The
metabolite that serves as the phosphoryl group acceptor for a specific kinase is identified by
the prefix of the kinase name. Hexokinase is non-specific as it is contained in all cells that
catalyze the phosphorylation of hexoses (D-Glucose, D-Mannose, D-Fructose etc.).
Phosphorylation of glucose has its cellular advantage as it keeps the glucose in the cell and also
keeps the glucose cellular concentration low to allow diffusion. At low glucose concentration
prevalent in most cells phosphorylation is achieved by the activities of hexokinase. While at
high glucose concentration in the liver phophorylation of glucose is achieved by the activities
of insulin inducible glucokinase.
Phosphoglucose Isomerase
The second reaction of the glycolytic pathway is the conversion of G-6-P to Fructose-6phosphate (F-6-P) phosphoglucose isomerase (glucose-6-phosphate isomerase). This is the
isomerisation of aldose to ketose. The reaction involves opening of ring, isomerisation and ring
closure.
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Phosphofructokinase
This is the second ATP utilization step of glycolysis. The enzyme phosphofructokinase1(PFK-1) phosphorylates F-6-P to yield Fructose-1, 6-bisphosphate (F-B-P). PFK plays a
central role in glycolysis as it catalyzes one the pathway’s rate limiting steps.
Aldolase
Aldolase catalyzes the fourth reaction of glycolysis resulting in the cleavage of F-B-P to two
triose sugars glyceraldehydes-3-phosphate (GAP) and Dihydroxyacetone phosphtate (DHAP)
in an aldo cleavage reaction.
Triose Phosphate Isomerase
One of the products of aldo cleavage reaction GAP continues along the glycolytic pathway.
However, the DHAP and GAP are ketose-aldose isomers just as F-6-P and G-6-P. The
interconversion occurs by the actions of triose phosphate isomerase via an intermediate and
marks the end of the preparative phase of glycolysis.
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Glyceraldehydes-3-Phosphate Dehydrogenase (GAPDH)
This if the beginning of the payoff phase of the pathway resulting in the generation of high
energy intermediates. In the presence of NAD+ and Pi, GAPDH catalyzes the oxidation and
phosphorylation of GAP to 1, 3-Bisphosphoglycerate (1, 3-BPG) with concomitant generation
of NADH. In this reaction the exergonic oxidative reactions drives the synthesis of the acyl
phosphate 1, 3 BPG.
Phosphoglycerate Kinase (PGK)
This is the step for the first synthesis of ATP (substrate level phosphorylation) in the glycolytic
pathway. In this reaction the phosphate moiety attached to position one of 1, 3 BPG is
transferred to ADP to generate ATP and 3-phosphoglycerate (3-PG) by the activities of PGK.
Phosphoglycerate Mutase (PGM)
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In the eight reaction of glycolysis, 3-PG is converted to 2-phosphoglycerate (2-PG) by PGM.
The mutase enzyme catalyzes the transfer of functional groups from one part of a molecule to
another. The phosphate group at carbon 3 of 3-PG is transferred to carbon as seen in 2-PG.
Enolase
In the reaction catalyzed by enolase, 2-PG is dehydrated to generate phosphoenol pyruvate
(PEP). The product of enolase activity PEP is the second high energy intermediate of the
glycolytic pathway.
Pyruvate Kinase (PK)
This is the last reaction of the glycolytic pathway and the second point of ATP generation
(substrate level phosphorylation). The PK couples the free energy of hydrolysis of PEP to
synthesis of ATP and formation of pyruvate, the product of glycolysis.
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REGULATION OF THE PATHWAY
Since a metabolic pathway is a series of enzyme-catalyzed reaction, there exist flux control
mechanisms that regulate the pathway. Most glycolytic reactions are reversible but three of
them are markedly exergonic and considered physiologically irreversible. These reactions are
catalyzed by hexokinase, PFK and PK and constitute the major site for regulation of glycolysis.
The rate of glycolysis must be regulated to meet the needs of the intracellular and extracellular
requirements. The degradation of glucose to pyruvate is regulated to meet two major cellular
needs which include; ATP generation and the provision of building blocks for synthetic
reactions, such as the formation of fatty acids. Most glycolytic reactions are reversible but three
of them are markedly exergonic and considered physiologically irreversible. These reactions
are catalyzed by hexokinase, PFK and PK and constitute the major site for regulation of
glycolysis. Each of the irreversible reactions serves as a control site. Their activities are
regulated by the reversible binding of allosteric effectors or by covalent modification. In
addition, the amounts of these important enzymes are varied by the regulation of transcription
to meet changing metabolic needs.
Phosphofructokinase is the most important control element in the mammalian glycolytic
pathway. High levels of ATP allosterically inhibit the enzyme in the liver (a 340-kd tetramer),
thus lowering its affinity for fructose 6-phosphate. The presence of AMP reverses the
inhibitory action of ATP, and so the activity of the enzyme increases when the ATP/AMP ratio
is lowered. The inhibition of phosphofructokinase by H+ prevents excessive formation of lactic
acid and a precipitous drop in blood pH.
Glycolysis
is
a
source
of
carbon
skeletons
for
other
biosynthetic
pathways.
Phosphofructokinase is inhibited by citrate, an intermediate in the citric acid cycle. Citrate
inhibits phosphofructokinase by enhancing the inhibitory effect of ATP.
In 1980, fructose 2,6-bisphosphate (F-2,6-BP) was identified as a potent activator of
phosphofructokinase. Fructose 2,6-bisphosphate activates phosphofructokinase by increasing
its affinity for fructose 6-phosphate and diminishing the inhibitory effect of ATP. Two
enzymes regulate the concentration of fructose 2,6-bisphosphate regulator of glycolysis by
phosphorylating fructose 6-phosphate and dephosphorylating fructose 2,6-bisphosphate.
Fructose 2,6-bisphosphate is formed in a reaction catalyzed by phosphofructokinase 2 (PFK2),
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a different enzyme from phosphofructokinase. Fructose 2,6-bisphosphate is hydrolyzed to
fructose 6-phosphate by a specific phosphatase, fructose bisphosphatase 2 (FBPase2).
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