cyanide
it's a reversible non-competitive inhibitor of cytochrome oxidase, an enzyme involved in cellular respiration.
it's molecular structure is very simple:
but what is a non-competitive inhibitor?
inhibitor = stops the action of (in this case, an enzyme)
non-competitive = does not compete with the substrate
a non-competitive inhibitor is not structurally similar to the substrate at all, so none of that molecular mimicry stuff happens.
it binds at a site away from the enzyme's active site, often called the allosteric site
by doing so, it causes a conformational change in the enzyme, which is basically like a chain reaction where the entire structure of the protein is affected by a very localized disturbance.
the active site is distorted and no longer complementary to the original substrate, so the substrate cannot bind.
in other cases, the active site remains intact but the catalytic portion of the active site, the few amino acids there that actually speed up the reaction, are thrown out of there and relocated someplace else. so the substrate can only bind but is forced to sit there uselessly until the end of time, because if there's no reaction, there's no product, and the substrate will remain stuck in the enzyme forever.
more on the catalytic portions of active sites. they speed up reactions by:
putting reactants in the correct orientation (this is for anabolic reactions, where anabolic = building up from smaller pieces)
weakening bonds in the reactant to reduce activation energy of reaction
acting as a "courier" for reactants that are oppositely charged and would normally repel each other (or like a couple's therapist)
allowing close proximity of the reactants, holding them together to give them a higher chance of reacting
providing a water-free zone using hydrophobic (water hating) amino acids, so that non-polar reactants can react more easily
for more on cyanide's action on cytochrome oxidase, there's a cool study from 1952 using beef as a source of the enzyme:
