I'm bringing over some conversation from a thread I started here.  It appears that thus far this thread has been limited mainly to discussions of engineering, bacteria/yeast, etc., whereas the topic I'd begun digging into there had mainly to do with those enzymes already identified, kinetic constants and all, in our friend phalaris.

Here is one quote from that thread, where I summarize the detailed account of phalaris' enzymatic activity as it is given in this paper.