The best way to study Biochemistry is by using First Aid. Some people have also mentioned that they find the Lippincott book very useful. Overall, I would say that you are most definitely fine JUST using First Aid, and that anything additional is just for your paranoia lol.
That said, Biochemistry routinely comes in between 4-7% of the exam. That includes hardcore mechanism and metabolism questions (Electron Transport, Krebs Cycle, Glycolysis, NADH stuff, etc.).
Check out these videos for a high yield jump on some important topics