Sarin blocks an important enzyme which hydrolyzes esters. When I look at its structure, I see the makings of a transition-state analog for the hydrolysis of a carboxylic acid ester:
An enzyme catalyzes a simple displacement reaction:
A—B + C ---> A—C + B
Think of a couple divorcing or swapping partners. A begins bonded to B but winds up bonded to C and B is freed from bondage to A. A is the central player--always bound to at least one other. Now there is a crucial moment (not shown but implicit in the reaction arrow, -->) when A simultaneously binds to both B and C; it's the unstable moment before A fully lets go of B to fully clutch C. It's an ephemeral moment or "state," requiring A to partially bind one more atom than it's used to doing: B--A--C has an awkward, fleeting existence. Because A ultimately binds C more strongly than B, A will momentarily contort "uphill" to ultimately wind up more stable with C.
The enzyme's job (or more generically, the catalyst's job) is to stabilize the awkwardness of the transition state: B--A--C. In the divorcing or partner swapping analogy, the social milieu, (acceptance, enablement) will facilitate the transition. In the absence of a catalyst, the displacement would face a much higher barrier.
Back to sarin. The sarin molecule closely resembles the transition state for the normal enzymatic process. Sarin looks just like hypothetical "B--A--C" transition state. Sarin goes into the enzyme and monkey wrenches the whole process: A—B never gets a chance to react with C because sarin comes in and blocks the whole transaction. And Sarin sticks like glue to the enzyme. Life processes end.