[burnte]

Proteins are just groups of amino acids which occur in stellar nebulae. It could easily be a widely used building block of life.

[maxerickson]

I'm saying they can attack an alien just as likely as they can a human.

For that to be true, you need some governing rule of life, not 'could easily be a widely used'.

(You'll note, I phrased my other reply in terms of us not knowing)