Why do you think that matters? Many things have common functional groups, and in many cases, there are no similar molecules. Functionality doesn't transfer that way (it's a highly non-linear process).
As for similar reactions- well, that's nice, but again, just seeing that a molecule participates in some chemistry has little to no predictive ability for health outcomes.
"Ability to replace calcium in certain biochemical processes" is not nearly the same as "chemically similar to calcium". It's like saying if I stole your login for facebook I would look like you and nobody would be able to distinguish between us without DNA test. Completely different things. Biochemical processes are known for accepting a lot of substitutes, that's how a lot of drugs work. That does not mean all those are chemically similar in general - they just play similar roles in certain processes.
What do you mean by "chemically similar"? I am not aware of this having a meaningful definition when working with toxins, and ... well, I have a PhD in the field.
One caveat, though: the identity of chemicals needs to be pretty carefully defined. Especially any time chiral things are involved, naturally-derived things are much more likely to be a pure enantiomer or close to it while artificial synthesis is more likely to produce racemates unless you are very careful.
> If we lived in a nonorientable world, these chemicals would be indistinguishable.
True, but you might have to move such a molecule arbitrarily far ("all the way around the Mobius strip") in order to change it from one enantiomer to ther other.
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