The theoretical maxium for kcat/Km is about 108 to 109 (mol/L)-1s-1. At this point, every collision of the enzyme with its substrate will result in catalysis. Some enzymes, such as fumarase, actually approach this limit.

My questions are:

1) How do enzymes manage to grab substrates from the solution they are in. Is there a harpoon mechanism present?

2)How can fumarase have such a high enzyme efficiency, it seems that steric factors are neglected or is an exageration that EVERY COLLISION will result in catalysis. Please explain why.