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Glycolysis

Written by tutor America N.

The first step in the process culminating in ATP synthesis by the mitochondria is glycolysis, or the anaerobic breakdown of glucose into pyruvate. Glycolysis is a 10-step process that takes place outside of the mitochondria, in the cell cytosol, and is catalyzed by a series of enzymes.

Glucose is first phosphorylated by ATP; this reaction is catalyzed by hexokinase using Mg2+ as a cofactor, and also converts the ATP molecule into ADP. The resulting glucose 6-phosphate is then reversibly converted into fructose 6-phosphate by the enzyme phosphoglucose isomerase. Fructose 6-phosphate is phosphorylated by a second molecule of ATP, forming fructose 1,6 biphosphate and an ADP. This reaction is catalyzed by phosphofructokinase using Mg2+ as a cofactor, and is largely irreversible.

The next step in glycolysis, catalyzed by fructose biphosphate aldolase, produces two products: glyceraldehyde 3-phosphate, which continues in the glycolysis pathway, and dihydroxyacetone phosphate, which is converted to glyceraldehyde 3-phosphate by triose phosphate isomerase. There are two molecules made of these intermediates for every molecule of glucose that enters glycolysis; in fact, this marks the step where the six-carbon skeleton of the glucose molecule is cleaved, and the reactions continue to yield two molecules of product (not pictured in diagram).

Glyceraldehyde 3-phosphate is then phosphorylated on carbon 1 by glyceraldehyde phosphate dehydrogenase using Mg2+ as a cofactor, forming 1,3-biphosphoglycerate. NAD+ and Pi aid in catalyzing this reaction, and NADH and H+ atoms are thus formed. The next step in glycolysis dephosphorylates 1,3-biphosphoglycerate, forming a molecule of ATP (two total, not pictured in diagram). Phosphoglycerate kinase is the enzyme that catalyzes this process, using Mg2+ as a cofactor, and 3-phosphoglycerate is the resultant product. This 3-phosphoglycerate is reversibly converted into 2-phosphoglycerate by the enzyme phosphoglyceromutase. 2-phosphoglycerate is dehydrated by enolase, which uses Mg2+ as a cofactor, forming phosphoenolpyruvate and two molecules of water. Phosphoenolpyruvate is then dephosphorylated, forming pyruvate and ATP. This reaction is catalyzed by pyruvate kinase, using Mg2+ as a cofactor. This pyruvate is the final product of the glycolysis pathway, along with a net gain of 2 ATP.

Glycolysis Flow Chart Image

Glycolysis Practice Quiz

Now, test your knowledge by answering the questions on glycolysis!

True or false: There are 5 reactions in glycolysis that use Mg2+ as a cofactor.

A. True
B. False
The correct answer here would be B.

There are six reactions in glycolysis that use Mg2+ as a cofactor.

What enzyme catalyzes the reversible conversion of glucose 6-phosphate to fructose 6-phosphate?

A. Hexokinase
B. Phosphoglucose isomerase
C. Enolase
D. Pyruvate kinase
The correct answer here would be B.

Phosphoglucose isomerase catalyzes the conversion of glucose 6-phosphate to fructose 6-phosphate.

Where does glycolysis take place in the cell?

A. Inside the mitochondria
B. Inside the Golgi complex
C. In the nucleus
D. In the cell cytosol
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