Cryptic epistasis in the evolution of enzyme mechanism, structure, and dynamics

Speaker: Douglas Theobold, Associate Professor at Department of Biochemistry, Brandeis University, Waltham Massachusetts, U.S.A.
Host: Thomas Hamelryck, Computational and RNA Biology

Abstract
Most biochemical reactions take from hundreds to billions of years to occur spontaneously. However, life depends on highly organized networks of catalyzed chemical reactions that proceed not only rapidly, but specifically and with high fidelity. Biological catalysts are enzymes, complicated molecular nanomachines that massively accelerate reactions by positioning specific substrate molecules with such precision that they are compelled to react. The molecular mechanism by which an enzyme executes this remarkable feat involves an exquisitely orchestrated sequence of steps. The structures, mechanisms, and functions of enzymes are all products of millions of years of evolution. Yet despite their fundamental biological importance, we have only a rudimentary understanding of the atomistic basis of the evolutionary changes that create novel enzymes.

In my lab we elucidate, at an atomistic level of description, the biophysical principles that underlie the evolutionary changes in structure, dynamics, and mechanism producing novel enzymatic functions. We resurrect entire evolutionary lineages of ancestral enzymes, solve their structures, characterize their dynamics, and determine their kinetic mechanisms, all correlated with the functional changes observed along these evolutionary trajectories. One of our main model systems is the malate and lactate dehydrogenase (M/LDH) superfamily. Both enzymes are found in the core metabolism of nearly every organism on the planet. M/LDHs are homologous enzymes that share a fold and catalytic mechanism yet can possess extraordinarily strict specificity for their substrates. The evolution of this family is marked by many important functional innovations, including (1) sharp alterations in substrate specificity, (2) changes in catalytic rate, (3) gain of allosteric control by small effector molecules, (4) acquisition of thermophilic, cryophilic, halophilic, and alkalophilic stability, and (5) the evolution of multimerization via new protein-protein interfaces. Many of these novelties are convergent, having evolved several times independently.

How do substitutions far from the active site affect activity? What is the molecular basis of epistasis? Does specificity increase during evolution? Were the ancestors of M/LDHs promiscuous? By answering these questions, we are providing the first fine-grained description of how enzyme structures and kinetic mechanisms constrain and channel genetic evolutionary processes.