Glycolysis
Glycolysis is a metabolic pathway that breaks down glucose
into pyruvate, producing energy in the form of ATP (adenosine triphosphate) and
NADH (nicotinamide adenine dinucleotide). This process occurs in the cytoplasm
of both prokaryotic and eukaryotic cells and is the first step in both aerobic
and anaerobic respiration.
The process of glycolysis can be divided into two phases:
the energy investment
phase and the energy payoff phase.
The energy investment phase begins with the phosphorylation
of glucose to produce glucose 6-phosphate, which is catalyzed by the enzyme
hexokinase.
This reaction
requires the input of one ATP molecule and results in the production of ADP.
Glucose 6-phosphate is then converted to fructose 6-phosphate by the enzyme
phosphohexose isomerase. Fructose 6-phosphate is then phosphorylated again by
ATP to produce fructose 1,6-bisphosphate, catalyzed by the enzyme
phosphofructokinase.
This reaction requires the input of a second ATP molecule
and also results in the production of ADP.
The energy payoff phase begins with the cleavage of fructose
1,6-bisphosphate into two three-carbon molecules:
dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.
IN This reaction is catalyzed by the enzyme aldolase.
Dihydroxyacetone
phosphate is converted to glyceraldehyde 3-phosphate by the enzyme triose
phosphate isomerase.
Glyceraldehyde 3-phosphate is then oxidized by the enzyme
glyceraldehyde 3-phosphate dehydrogenase, which transfers electrons to NAD+ to
produce NADH.
This reaction also results in the production of a
high-energy intermediate, 1,3-bisphosphoglycerate. The high-energy phosphate
group in 1,3-bisphosphoglycerate is then transferred to ADP to produce ATP,
catalyzed by the enzyme phosphoglycerate kinase.
This reaction
produces 3-phosphoglycerate.
The final steps of glycolysis involve the conversion of
3-phosphoglycerate to pyruvate. First, 3-phosphoglycerate is converted to 2-phosphoglycerate
by the enzyme phosphoglycerate mutase. 2-phosphoglycerate is then converted to
phosphoenolpyruvate (PEP) by the enzyme enolase. PEP is converted to pyruvate
by the enzyme pyruvate kinase, which produces another ATP molecule by
transferring a high-energy phosphate group to ADP.
In total, the net result of glycolysis is the production of
two molecules of ATP, two molecules of NADH, and two molecules of pyruvate per
molecule of glucose.
In anaerobic
conditions, pyruvate is converted to lactate or ethanol, producing NAD+ that
can be used in glycolysis to continue producing ATP.
In aerobic
conditions, pyruvate enters the mitochondria and is further oxidized through
the Krebs cycle and oxidative phosphorylation to produce more ATP.
Glycolysis is a highly conserved pathway that is found in
all living organisms. It plays a crucial role in energy metabolism, providing
the initial steps for both aerobic and anaerobic respiration.
In addition, glycolysis has other important functions,
including the production of precursors for the synthesis of amino acids and
nucleotides, and the regulation of gene expression through the production of
signaling molecules like fructose 2,6-bisphosphate.
Overall, glycolysis is a complex and highly regulated
pathway
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