I was recently asked to explain why [(fructose-bisphosphate) aldolase](https://en.wikipedia.org/wiki/Fructose-bisphosphate_aldolase) does not act on [glucose 6-phosphate](https://en.wikipedia.org/wiki/Glucose_6-phosphate) in glycolysis and waits until the formation of [fructose 1,6-bisphosphate](https://en.wikipedia.org/wiki/Fructose_1,6-bisphosphate). I did not know much about the enzyme structure but tried to put an unconvincing explanation on such grounds emphasizing the specificity of the substrate, which was rejected.

The instructor's answer was that such a reaction would have led to the formation of glyoxal and, [in turn](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5517486/), toxic advanced glycation end products (AGEs). Thus, such a reaction is precluded. I am not remotely convinced, notwithstanding selection pressure, and look forward to alternative explanations or constructive critiques of the very premise of the question.

**P.S.**: I did some digging in Google Scholar and came across [this old article](https://academic.oup.com/plphys/article/51/2/263/6072867), reporting on a cycloaldolase that acts upon glucose 6-phosphate to produce myo-inositol.

**P.P.S.**: Here's a [ready-reckoner flowsheet](https://commons.wikimedia.org/wiki/File:Glycolysis.svg) for the glycolysis pathway.