Overcoming barriers in structural biology through method development of serial crystallography

Serial crystallography (SX) is a relatively new structural biology technique that collects X-ray diffraction data from microcrystals via femtosecond pulses produced by an X-ray free electron laser (X-FEL) or by synchrotron radiation, allowing for challenging protein structures to be solved from microcrystals at room temperature. Because of the youth of this technique, method development is necessary for it to achieve its full potential.

Most serial crystallography experiments have relied on delivering sample in the mother liquor focused into a stream by compressed gas. This liquid stream moves at a fast rate, meaning that most of the valuable sample is wasted. For this reason, the liquid jet can require 10-100 milligrams of sample for a complete data set. Agarose has been developed as a slow moving microcrystal carrier to decrease sample consumption and waste. The agarose jet provides low background, no Debye-Sherrer rings, is compatible for sample delivery in vacuum environments, and is compatible with a wide variety of crystal systems. Additionally, poly(ethylene oxide) which is amenable for data collection in atmosphere has been developed for synchrotron experiments. Thus this work allows sample limited proteins of difficult to crystallize systems to be investigated by serial crystallography.

Time-resolved serial X-ray crystallography (TR-SX) studies have only been employed to study light-triggered reactions in photoactive systems. While these systems are very important, most proteins in Nature are not light-driven. However, fast mixing of two liquids, such as those containing enzyme protein crystals and substrates, immediately before being exposed to an X-ray beam would allow conformational changes and /or intermediates to be seen by diffraction. As a model, 3-deoxy-D-manno-2-octulosonate-8-phosphate synthase (KDO8PS), has been developed for TR-SX. This enzyme initializes the first step of lipopolysaccharide synthesis by a net aldol condensation between arabinose-5-phosphate, phosphoenol pyruvate, and water. During this reaction, a short lived intermediate is formed and has been observed on a millisecond timescale using other methods. Thus KDO8PS is an ideal model protein for studying diffusion times into a crystal and short mixing times (<10 ms). For these experiments, microcrystals diffracting to high resolution have been developed and characterized.