My Track Record    

(Part 1: S-adenosylmethionine decarboxylase from T. brucei.)

I've previously noted the awesome catalytic power of orotidine monophosphate decarboxylase (OMPDC), so I will give it position #2 on my random-order list of favorite enzymes. The impressive rate enhancement of OMPDC has been of practical value to me during the past two years. When sharing the Plasmodium falciparum OMPDC with other labs, we haven't had to send them very much because it's so active.

Speaking of Plasmodium -- the parasite that causes malaria -- the best treatments for infection involve derivatives of artemisinin, a natural product of the plant Artemisia annua. Jay Keasling and his coworkers and collaborators have spent much of the past decade figuring out how to make artemisinin in high yields using yeast and bacteria as chemical factories. The full story is complicated, but a key discovery along the way was that A. annua contains a novel cytochrome P450 monooxygenase, CYP71AV1, which catalyzes three consecutive oxidation reactions at the C12 position of amorpha-4,11-diene, converting a methyl group (-CH3) to a carboxylic acid (-COOH). The final product is known as artemisinic acid.

Though a three-step oxidation by a P450 enzyme is not unprecedented, it is very convenient and very cool that cloning and characterization of a single enzyme brought Keasling et al. three steps closer to their goal of inexpensive, high-yield production of artemisinin. CYP71AV1 has thus earned spot #3 on my favorite-enzymes list.

[Reference: D.-K. Ro et al. (2006). Production of the antimalarial drug precursor artemisinic acid in engineered yeast. Nature 440: 940-3. Figure 1 shows the oxidation steps mentioned above.]

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I've written before about Phil's interest in law enforcement and jail. His new favorite game incorporates both of these themes. The one-sentence summary is that he repeatedly hunts me down and throws me in jail. It doesn't sound like much, but the following photo gives some indication of his enthusiasm.

With his army outfit, NYPD Blue cap, binoculars, and baseball bat doubling as a gun, doesn't he look ready to keep the inmates subdued? The funny thing, though, is that once he puts me in jail, he immediately lies down and starts making loud snoring noises. This is my cue to escape so that the cycle can begin again. On a good day (or a bad one, depending on your perspective), I might bust out 15 or 20 times.

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For me, a highlight of the recent ICOPA XII conference was getting to hear Meg Phillips talk about her work on S-adenosylmethionine decarboxylase (EC 4.1.1.50) in Trypanosoma brucei. This enzyme is a part of the polyamine synthesis pathway and, as such, might be a good one to target in the treatment of African sleeping sickness.

When Meg's lab started to characterize the T. brucei AdoMetDC in vitro, they found that its activity was only about 1/300 of that the human enzyme. If I had been doing this study, I might have assumed simply that the assay conditions were somehow ill-suited to the T. brucei enzyme. But Meg's group dug deeper -- much deeper. They found that, in addition to the normal gene for AdoMetDC in the T. brucei genome, there was a second AdoMetDC-like gene. This gene codes for a "prozyme" that is not itself catalytically active, but that binds to the "real" AdoMetDC and allosterically activates it, boosting its activity by 1,200-fold. Thus, in live T. brucei cells, the two polypeptides active form a highly unusual but highly effective heterodimer. Even after hearing this story a couple of times, I remain amazed that Meg's lab figured it out.

[Reference: E. K. Willert, R. Fitzpatrick, and M. A. Phillips (2007). Allosteric regulation of an essential trypanosome polyamine biosynthetic enzyme by a catalytically dead homolog. PNAS 104: 8075-80.]

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