I think that blocks cytochromes, though I think before that you are killed by its blocking haemoglobin, which is often called an 'honorary enzyme' anyway. What does work by blocking cytochromes is cyanide.
Then everyone should have some awareness of the organophosphorus compounds, some the notorious nerve gases, others insecticides, which act by reacting covalently with acetyl cholinesterase so blocking transmission of nervous impulses. Curare on the other hand is a a blocker of acetylcholine receptors - and surprisingly in view of the previous an antidote such as physostigmine is also an acetylcholinesterase inhibitor - which works by letting acetylcholine build up to overcome the curare, but unlike the organophoshates is bound reversibly
OK the receptor is a protein controlling an ion gate, but between such physical-chemical and strictly chemical actions you find yourself not making distinction when you get into such areas.
I do not remember an example but some toxic substances not toxic as such but are transformed enzymatically into other substances that are. And then there are lots of enzymes that do the opposite and de
toxify substances, oxidise them, methylate them, and I don't remember what else. Quite a big and practical area.
Here is another enzyme with complicated action like those mentioned by Jon http://www.ebi.ac.uk/interpro/potm/2005_9/Page2.htm