Reaction 5 of Phase A of glycolysis represents the final reaction for Phase A of glycolysis. In it, it represents an isomerization reaction. So here, we're going to say dihydroxyacetone phosphate, which we abbreviate as DHAP, is isomerized to Glyceraldehyde 3-phosphate, which we abbreviate as G3P. Now, this is catalyzed by the enzyme triosephosphate isomerase, and because we're dealing with isomerization, it would be an isomerase. So, if we take a look here, we have dihydroxyacetone phosphate that isomerizes into Glyceraldehyde 3-phosphate. Since it's a reversible reaction, it can go the opposite way as well, where we're going from G3P to DHAP.
Now, here, dihydroxyacetone phosphate contains an acetone, which is a ketone, meaning that this carbon in the middle would have to be a ketone carbon. And then here, Glyceraldehyde, as the name suggests, contains an aldehyde. Remember, an aldehyde is a carbonyl group, so C double bond O, connected to an H. This explains how we are going between these two different molecules because it's an isomerization reaction; we use an isomerase to do it. Remember, this represents the final reaction, Reaction 5 of Phase A of our glycolysis.