In general for biochem, know that when your missing an enzyme, everything before it will "back up" (higher serum values) and everything after it will decrease. Know the starting enzyme, the end product (what is the point of the cycle), and the intermediates that World is quizzing you on. You don't have to memorize every substrate. Known the rate limiting enzymes as well. I am not the best in biochem, but I was told that if you know it well it's one of the easier subjects. I find that hard to believe..haha.