T2i change scroll wheel to Av instead of shutter speed?

[pictonaut]

Hi all,

sorry if this has been answered in any way, but i used the search and nothing came up. basically, i'd like to know if i can change the scroll wheel / main dial on my 550D to Av instead of shutter speed. The lock shutter function is already cool, but I'd find it much more practical than to have to hold the Av button at the same time . Am I missing an obvious function or can it be done?

Cheers!