Que Serine, Serine

Sarin gas arouses fear and outrage because it works so silently and efficiently. I don't mean to flatter it by using the word "efficient" but it's really much more deadly and lethal than anything fielded in WW I. And remember it was discovered by accident. I doubt anyone could have designed something that efficient back then: nearly all of our understanding of how sarin kills came after its discovery, including the basic notions of how enzymes work. If someone survives a sarin exposure, there's the secondary problem of getting rid of it in cells. It's the tenacity of a simple phosphate bond — the same type of linkage found elsewhere including DNA and RNA — that keeps it from getting flushed out.

Wiki Link

The cartoon depicts the active site in an enzyme called acetylcholinesterase found in nerve and muscle cells. Notice the little red ball in the concave region called "esteratic site." The red ball is called a serine hydroxyl and is a metaphorical bond cleaver. 

Acetylcholine is a neurotransmitter. Our cells make and destroy these chemical messengers over and over in order to control nerve impulses. The enzyme cleaves acetylcholine when and where it is no longer needed. It is recycled elsewhere where it is needed. 

The top cartoon labeled "normal" shows acetylcholine approaching the enzyme, getting cleaved, and then getting washed away, regenerating the enzyme's active site. This all happens in microseconds. 

The second panel called "reversible inhibitor" shows a less deadly reversible inhibitor and how the rate of washing away (hydrolysis) lasts for minutes, but regenerates the serine cleaver. 

The third panel shows a molecule which resembles sarin. In a nutshell, the problem with sarin (besides its being a gas) is that it delivers a reactive phosphate which makes a very strong P—O linkage, sticking like glue to the serine cleaver.

Here is a YouTube video created by a chemist to try and explain things in more detail: link.  I tried saying the same thing in words here, but words fail.