Istanbul,
Welcome aboard!!
I started riding @ 330. I've had very few weight related problems with my bike. True, the wheels DO need a bit more attention, but so far, so good. Actually, my top weight, in 2001 was 424. My wife and I joined Weight Watchers, and have not looked back. Between us we've LOST 245 pounds. (152 for me, 93 for her)
We feel incredible!! You won't regret it. I agree with consulting with the doctor first. He may tell you to take it easy on the bike until you get your weight down somewhat. (mine did)

BTW, you WILL look kinda funny on your bike, and rude people WILL probably make comments to you about it, but SO WHAT. They don't get it, never will. You do what YOU gotta do. You can improve yourself, those types will ALWAYS be idiots.