I wouldn't say 'the speed of chemical reactions' - rather say 'the rate of chemical reactions'. Assessors strongly consider the use of proper scientific language when marking.
Also, consider mentioning that each enzyme is specific to only one substrate (assessors love it if you use the word 'specificity' correctly).
I think the depth of my answer will ultimaty depend on the bumber of lines on an exam/SAC. The thing is, I'd mention my definition for a 2line answer whereas a 4 line question (i.e. 4 lines to write on), I would mention the following
* an organic, protein catalyst
* increases rate of chemical reactions by lowering activation energy
* the tertiary structure of the enzyme is critical to its specific function as the shape of the active site is specifically complementary to its substrate!
^ I think that would be satisfactory for an in depth answer they may very-well ask.