The MCAT is designed so that supposedly you can take the exam with just the bare minimum premed requirements: 2 semesters of biology, 2 of gchem, 2ochem, and 2 physics....that's it. There are biochemistry topics covered in the mcat, but these are the topics that are supposed to be covered in first semester of biology and organic, like configurations of sugars and carbonyl chemistry, mostly macromolecules and basic metabolism (glycolysis, Krebs, etc.). A full biochem course is not really needed for the test (although it should be taken for matriculation in med school, and some schools require it) although it does help you understand the topics for the test better. If you have time to take it before the test, I would, otherwise just take it before you graduate. Upper level courses are not needed for the test but can help tremendously.