Bioenergetics & Regulation of Glycolysis

A note on the bioenergetics (ATP production) and regulation of glycolysis.

1. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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Bioenergetics & Regulation of Glycolysis
Glycolysis is the breakdown of glucose into pyruvate / pyruvic acid. It is the universal
central pathway of glucose catabolism. It occurs in the cytosol in both aerobic and
anaerobic conditions. It has 10 steps in total, each catalyzed by a specific enzyme. In
addition to enzymes, a nucleotide triphosphate ATP (adenosine triphosphate) and a
coenzyme NAD
+
(nicotinamide adenine dinucleotide) are also required.
Bioenergetics of Glycolysis
In the preparatory phase of glycolysis, 2 ATP molecules are used in the phosphorylation
of glucose. The first is used when glucose is phosphorylated into glucose 6-phosphate:

The second is used when fructose 6-phosphate is phosphorylated into fructose 1,6-
bisphosphate:

2. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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In the payoff phase, no ATP molecules are used, instead 4 ATP molecules are
generated and 2 NAD+ molecules are reduced to 2 NADH.
NADH is produced when glyceraldehyde 3 -phosphate is phosphorylated to 1,3-
bisphosphoglycerate (inorganic phosphate is used and not ATP). One NADH is
produced for one molecule of glyceraldehyde 3-phosphate (G3P), but since two G3P
are produced from one glucose molecule, 2 NADH are gained:

The first ATP producing step in glycolysis is the phosphoryl transfer from 1,3-
bisphosphoglycerate to ADP. One ATP is produced from one molecule of 1,3 -
bisphosphoglycerate, but since there are two of them, two ATP molecules are hence
produced.
The second ATP producing step is the transfer of the phosphoryl group from
phosphoenolpyruvate (PEP) to ADP. Since there are two molecules of PEP (produced
from one molecule of glucose), 2 ATP are produced.

3. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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Two ATP are produced because of each of the above reactions (left and right), making
a total of four. But two ATP were also used in the preparatory phase of glycolysis.
Hence there is a net gain of 2 ATP and 2 NADH (since no NADH molecules were used).
This gain can be expressed as follows:

The net gain of glycolysis is pyruvate (product) and: 2 ATP + 2 NADH.
Regulation of Glycolysis
There are three regulatory enzymes for glycolysis: (1) Hexokinase & glucokinase
(isozymes), (2) Phospho-fructokinase, and (3) Pyruvate kinase. All three of these

4. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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enzymes catalyze the only three irreversible reactions of glycolysis (the rest are
reversible). Hormones also affect the rate of glucose breakdown.
1. Hexokinase & Glucokinase:
Hexokinase catalyzes the first step of glycolysis, the conversion of glucose to glucose 6-
phosphate (G6P). Glucokinase (an isozyme of hexokinase) converts glucose into
glucose 1-phosphate (G1P). G1P is not used in glycolysis, but rather it is used to make
glycogen (glycogenesis), fat, cholesterol, etc. High levels of G6P inhibit hexokinase
(negative feedback) but do not affect glucokinase. Therefore, high levels of G6P cause
the cell to store excess glucose and inhibit glycolysis. When G6P levels drop,
hexokinase activity resumes.
2. Phospho-fructokinase:
Phosphofructokinase (PFK) controls the third step of glycolysis, the conversion of
fructose 6-phosphate (F6P) into fructose 1,6-biphosphate (F1,6BP). PFK is inhibited by
high levels of ATP, low pH levels, and high levels of citrate, a byproduct of cell
metabolism. On the other hand, high levels of adenosine monophosphate (AMP) and
fructose 2,6-bisphosphate (F2,6BP) activate PFK and increase the rate of glycolysis. It
is the most important enzyme in glycolysis regulation.
3. Pyruvate Kinase:
Pyruvate kinase (PK) catalyzes the final step of glycolysis, the conversion of
phosphoenolpyruvate (PEP) into pyruvate. This step increases the concentration of ATP
in the cell, and high ATP levels inhibit PK (negative feedback). Fructose 1,6-
bisphosphate (F1,6BP), the product of glycolysis step 3, activates PK.
Role of Hormones in Glycolysis Regulation
Hormones also play an important role in glycolysis regulation. Some hormones promote
glycolysis, while others inhibit it. Insulin promotes glycolysis by activating hexokinase, to
reduce sugar level in blood. Glucagon, on the other hand, inhibits glycolysis and

5. SYED MUHAMMAD KHAN ( BS HONS. ZOOLOGY)

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promotes glucose synthesis (gluconeogenesis), it inhibits phospho-fructokinase.
Thyroxine and epinephrine (adrenaline) also promote glycolysis, but glucocorticoids
(cortisol) inhibit it.