A whole comprises its parts:

• “12 different enzymes comprise the system.” WRONG
• “The system comprises 12 different enzymes.” RIGHT

while the parts constitute the whole:

• “The system constitutes 12 different enzymes.” WRONG
• “12 different enzymes constitute the system.” RIGHT

The word compose is often used in place of comprise, though like constitute, the parts compose the whole, not visa versa. But by using the “is composed of” construction, the roles of the subject and object are reversed:

• “The system is composed of 12 different enzymes.” RIGHT

In my experience, this idiom is more common, especially in spoken English; probably due to the confusion about the proper use of comprise.

[Edit: due to broken CSS, wrong sentences were not properly marked wrong. Fixed.]