Aug 302008
 
ResearchBlogging.orgIt is rare, but not unheard of, for a human baby to be born with a tail. Atavism of this kind is generally understood to be the result of mutations in regulatory genes that cause an ancestral pattern of development to re-emerge. A physiological step backwards through the path of descent is often easy to recognize, because many of the evolutionary relationships are known. It should also be possible to identify atavistic events in particular molecules. For instance, one can imagine that a mutation to CLC-0 might result in a reversion to the ancestral transporter function. In a recent article in PLoS Biology, researchers from Florida State University and Brandeis University identify just such a relationship in the bi-functional enzyme inosine monophosphate dehydrogenase (IMPDH). PLoS Biology is an open-access journal, so open it up and follow along.

IMPDH plays a critical role in the synthesis of guanine nucleotides, an essential component of DNA. Two reactions take place in the active site — first, the inosine ring is oxidized to xanthosine, forming a covalent linkage with the enzyme, and then this bond is broken by a hydrolysis. The enzyme active site changes shape to carry out the reaction, bringing a catalytic arginine (R418) into position to activate the water for nucleophilic attack. Any time you see a complicated mechanism like this, it’s natural to wonder how such a system could have evolved. Min et al. performed simulations and experiments to find out.

Using a crystal structure of IMPDH as a starting point, Min et al. performed hybrid QM/MM simulations in which the atoms taking direct part in the reaction were treated with quantum mechanics, and the rest of the protein was simulated using molecular mechanics. As one would expect given the enormous reduction in catalytic rate that occurs when R418 is mutated, the reaction proceeded through the arginine when the simulation had a neutral R418 side chain. The water is stabilized by two additional side chains from T321 and Y419, and reacts almost instantaneously, without the formation of a stable hydroxide intermediate. Although this is unusual, this prediction of the simulation is consistent with isotope effect experiments.

When the arginine was replaced by a glutamine in the simulation, the mechanism changed, naturally. Under these conditions, it was Y419 that activated the water for the hydrolysis, although the energy barrier was much higher (leading to a slower reaction). Again, the characteristics of the reaction indicated by the simulation line up pretty well with the results of biochemical experiments. Of course, Y419 enters the active site the same way R418 does, so the question of how the hydrolase activity could have evolved remains open.

Something very interesting, however, happens when the simulation is performed with R418 in a charged state. A fully protonated arginine will have a very hard time activating water for a nucleophilic attack. The simulation indicated that under these conditions, T321 performed this role, after being activated by a nearby glutamate (E431). T321 is adjacent to cysteine 319, which is essential for the oxidation reaction, and is not located on the mobile flap. If T321 really can catalyze hydrolysis, this would mean that it is possible that IMPDH possessed an (inefficient) hydrolysis activity before it evolved the mobile flap.

Because T321 only plays a signficant role in catalysis when R418 is protonated, blocking this pathway should result in decreased IMPDH activity at low pH. This is precisely what Min et al. observe in enzymatic assays (Figure 5) on a mutant in which E431 is mutated to glutamine. There is other experimental support as well: IMPDH enzymes that have been mutated at R418 usually have large isotope effects, which makes sense in light of the fact that the alternative T321 pathway involves the simultaneous transfer of two protons (rather than just one).

Things get even more interesting when IMPDH is compared to one of its cousins, GMP reductase. Although GMPR catalyzes a very different reaction, the C319/T321/E431 triad is also present there. This, along with other data from sequence alignment, suggests that these three residues were also present in a similar configuration in the ancestor of these modern proteins. Over time, progressive optimization of the two proteins resulted in the T321 pathway being supplanted by the more effective R418 in IMPDH, while remaining essential in GMPR.

If T321 really is a remnant of an earlier water-activating pathway, why is it conserved now that IMPDH has a much more efficient catalytic residue available? T321 is probably preserved because it stabilizes the water while it is being activated by R418. However, the other essential residue of that activating pathway (E431) is usually an inactive glutamine in eukaryotic forms of IMPDH (and some prokaryotes, as well). In these species the T321 activation pathway has been completely supplanted by the arginine pathway. Yet in the other forms of IMPDH this alternative mechanism still lingers, perhaps because of the additional activity it affords at low pH, or because it confers resistance to a particular inhibitor of the enzyme. In that sense, IMPDH’s “tail” might provide an adaptive advantage quite different from that which gave rise to hydrolytic activity in the first place.

Donghong Min, Helen R. Josephine, Hongzhi Li, Clemens Lakner, Iain S. MacPherson, Gavin J. P. Naylor, David Swofford, Lizbeth Hedstrom, Wei Yang, Daniel Herschlag (2008). An Enzymatic Atavist Revealed in Dual Pathways for Water Activation PLoS Biology, 6 (8) DOI: 10.1371/journal.pbio.0060206 OPEN ACCESS
Disclaimer: Although I have little contact with Dr. Hedstrom’s group, I am also working at Brandeis.

 Posted by at 7:30 PM

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