What is Aldolase? - Definition, Mechanism & Reaction

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Aldolase is one of the enzymes involved in glycolysis, acting in step 4 of glycolysis. It breaks the fructose molecule into two smaller molecules, a dihydroxyacetone and a glyceraldehyde.

Aldolase Defined

Most things that you eat have at least some glucose in it. And if you happened to not eat enough glucose, then your body would start converting other compounds into glucose.

Glucose is one of the most basic sources of energy for the body. Getting from glucose to energy, though, takes several steps. These steps make up glycolysis, the pathway that breaks down glucose into pyruvate, which can be used to make energy.

One of these steps utilizes aldolase, the enzyme that converts fructose 1,6-biphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. This conversion happens in the 4th step in the glycolysis pathway, which splits glucose into two different molecules, enabling glycolysis to make 2 pyruvate molecules.

Aldolase Active Site and Mechanism

The main amino acid residue in the aldolase active site is lysine 223. Other amino acids residues act as a base or acid to help the reaction proceed. Lysine 97 and arginine 295 act as bases while glutamic acid 178 acts as the acid.

Aldolase is a large enzyme. The active site utilizes four amino acid residues: two lysine, one glutamic acid, and one arginine
Aldolase enzyme and active site

Let's talk about the aldolase mechanism. In the fist step, the open ringed fructose enters the enzyme and binds to the lysine, with the oxygen taking a hydrogen from the glutamic acid:

Step 1

Next the arginine takes a hydrogen from lysine, forming a double bond between lysine and carbon and kicking off the oxygen as water:

Step 2

Then Lysine 97 takes the hydrogen from the oxygen, forming a carbon-oxygen double bond, breaking the carbon-carbon bond, forming a new carbon-carbon double bond, and breaking one of the carbon-nitrogen bonds:

Step 3

At this point the first product, glyceraldehyde 3-phosphate, is formed, and leaves the enzyme:

Glyceraldehyde 3-phosphate

In the fourth step, the lysine reforms the nitrogen-carbon double bond, breaks the carbon-carbon double bond, and takes the hydrogen from arginine:

Step 4

Steps 5 and 6 are a Schiff base reversal. In the 5th step, water is added, breaking one of the nitrogen-carbon double bonds and adding a carbon-oxygen bond:

Step 5

In the 6th step, oxygen forms a double bond with carbon. The nitrogen-carbon bond is broken, and the hydrogen is taken from the oxygen:

Step 6

Finally, the second product, dihydroxyacetone phosphate, is released:

Dihydroxyacetone phosphate

The active site can reform by rearranging the hydrogens:

Active site reform

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