Gregory Petsko is a biochemist who studies the proteins of the body and their biochemical function. Working with Dagmar Ringe, he's doing pioneering work in the way we look at proteins and what they do. Gregory Petsko's own biography, on his Brandeis faculty homepage, might seem intimidatingly abstruse to the non-biochemist -- he studies "the structural basis for efficient enzymic catalysis of proton and hydride transfer; the role of the metal ions in bridged bimetalloenzyme active sites; direct visualization of proteins in action by time-resolved protein crystallography; the evolution of new enzyme activities from old ones; and the biology of the quiescent state in eukaryotic cells." But for someone so deeply in touch with the minutest parts of our bodies, Petsko is also a wide-ranging mind, concerned about larger health policy issues. The effect of mass population shifts -- such as our current trend toward a senior-citizen society -- maps onto his world of tiny proteins to create a compelling new worldview.