Sorry to hear that you decided life sciences wasn't for you. ):
The Business Informatics requirements are found
here. Unfortunately it appears you need to complete the
Computer Science I required courses including the three maths which may be restricted to someone not in the Faculty of Engineering. You can take the three comp sci courses though. Beyond that, you are required to take Econ 1B03 and Econ 1BB3 as your electives, which you can do as a life sciences student (and indeed, a lot of LS students take them as "birds").
In regards to summer school, courses are offered on a year-by-year basis. You might not know whether or not a comp sci course will be offered until the 2014 Summer Course Timetable comes out. For transfer credits at other schools, check out the information/list of equivalent courses at other unis
here.
My suggestion to you is to get into contact with the academic advisors in the
Associate Dean of Science Office somehow (you can go for drop-in advising, phone them, email, etc.) and explain your situation to see what they can do for you.
I do urge you to make sure you think this through before committing, though. Science has many branches and many opportunities aside from medical school. You say you don't think you could stick it through graduate school but an M. Sc. on average takes ~2 years to complete compared to the 3-4 years you would spend in medical school (not to mention residency).
Biochemistry as a whole is a lot more than just mouse-work or protein analysis. Research can go in many directions, and you don't necessarily have to do it in a university. Large pharmaceutical companies also employ scientists (as well as other opportunities for science graduates).
Granted, I'm biased (science nerd in Biochem - Biomed), but science has so much more to offer than just a means to the end of medical school. If you like science, then don't let the looming prospect of jobs or medical school or whatever get in your way.
That said, if you also have a passion for business informatics and feel that you would be happy there, go for it! Best of luck.