That's okay, it's just the word 'bothered' that took me aback a bit, but I'm over it. AFAIK, in general you don't put references to basic and easily verifiable information. (This is not an insult to the question, what's basic to someone is not to someone else, and you have to know enough to know how to phrase the question). If I were putting things that were only found in a handful of sources, definitely I'd put a reference. I'm new to StackExchange, so maybe the bar is set lower than I'm used to? What do you think in my response most needs a reference?

There's virtually no difference between 0.0001 and 0.00011; cutoffs work in orders of magnitude. Depending on what I'm doing, sometimes I'll use 0.1, sometimes I'll use 1E-30 (0.000000000000000000000000000001).

Thanks for pointing that out, I wasn't clear in my resonse, I didn't mean carbons form long chains just with each other, but every long biomolecule chain involves carbon, there's no other element that's in every biomolecule chain bond).

@WYSIWYG, you are absolutely correct in that if carbon were not so abundant, there would be no life as we know it!

@The Last Word, all of that stuff is pretty easy to find via google (once you know what terms to look for), I'm sorry, I don't have the references handy, it's all pretty basic background knowledge to people who are familiar with these things. I took time out of my life to answer a question, I'm sorry if you feel I didn't do enough.

I searched within the pdb for protease inhibitor. Since the inhibitors themselves are not proteins, most of the search results are proteases with inhibitors.

plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0029‌​797