Imaging enzyme kinetics at atomic resolution
The 1958 Nobel prize to Beadle and Tatum for proposing, in the main, that each gene is responsible for a distinct enzyme is now seen as both foundational to molecular biology and genetics. Some genes for example, code for functional RNAs, while others code for non-enzymatic proteins such as collagen. Yet enzymes remain fundamental to life on earth, catalysing at least 5000 biochemical reactions. Enzymes can increase reaction rates by huge factors, from millions of years to milliseconds per event, so that, from meat tenderizer to washing powder, to muscle contraction, cargo transport in the cell, ion pumps, infection and digestion, no molecular machine is more fundamental to biological function than the enzyme [Spence and Lattman (2016), IUCrJ. 3, 228-229].
In a recent publication [Horrell et al. [(2016), IUCrJ. 3, 271-281] shed some light on the mechanism of the catalytic cycle by creating a kind of atomic resolution X-ray molecular movie. Horrell et al. soaked “large” crystals of recombinant copper nitrite reductase in sodium nitrite for an hour at room temperature before transferring them to a cryo-protectant and plunging into liquid nitrogen, to trap the structure of the room temperature complex. At this stage the reaction does not proceed because no reducing agent is present. The required electrons are provided by free radicals generated by the very X-rays used to image the structure.
The authors then used the Diamond synchrotron fitted with a new fast shutterless detector to obtain 45 low-dose Bragg diffraction datasets in 19s each from the same regions of the crystal. This interval spans the catalytic cycle of nitrite reduction, a vital process in agriculture and in the formation of the greenhouse gas N2O.
In short, the new fast synchrotron detectors, soon to be combined with diffraction-limited sources, may allow us to make movies of enzyme kinetics. If these studies can be used to assist in modelling the atomistic mechanisms for enzymatic catalysis, it may indeed be possible, using recombinant DNA, to develop new enzymes with specifically tuned properties, which will have enormous implications for pharmacology, drug treatment and food production.
