Read for comprehension. Test your comfort comprehending organic chemistry in the biochemical context. Phosphoglucose isomerase catalyzes the transformation of glucose 6-phosphate into fructose 6-phosphate, facilitating firstly ring opening then formation of the cis-enediolate intermediate (the enolate of both the glucose and the fructose forms), then ring closure after completion of proton transfer.

Don't stress about remembering every detail for the MCAT, although this will certainly be required in medical school. Work on your comfort with biochemical nomenclature and your ability to place discussions in the broader context.