Anything that changes the shape of the active site may reduce or eliminate enzyme function. In porphyria changes in enzymes are the key essential element in triggering the onset of porphyria symptoms. Receptor sites exist in tissues. Many forms of chemical communication happen, each has a receptor site. Each may be interfered with by compounds that either copy the chemical messenger or which change the receptor site so that the chemical messenger is not recognized when it arrives.
Receptor sites are very specific as to the size and shape of the molecule that is recognized.
This is how mistakes in chemical communication are eliminated. Various compounds can interfere with the action of hormones either by combining with the hormone so that it is no longer 'recognized' at its receptor site or by modifying the receptor site. Often porphyria patients do not realize the potential toxin problems that exist with metals. In porphyria the pathways between cell exposure and expression of toxicity are often noted in the PN that accompanies an acute attack.