Characterization and evaluation of photolabile (µ-peroxo)(µ-hydroxo)bis[bis(bipyridyl)cobalt caged oxygen compounds to facilitate time-resolved crystallographic studies of cytochrome c oxidase

Photolabile (µ-peroxo)(µ-hydroxo)bis[bis(bipyridyl)-cobalt-based caged oxygen compounds have been synthesized and characterized by optical absorbance spectroscopy, X-ray crystallography. and the quantum yield and redox stability were investigated. Furthermore, conditions were established where redox incompatibilities encountered between caged oxygen compounds and oxygen-dependant cytochrome c oxidase (CcO) could be circumvented. Herein, we demonstrate that millimolar concentrations of molecular oxygen can be released from a caged oxygen compound with spatio-temporal control upon laser excitation, triggering enzymatic turnover in cytochrome c oxidase. Spectroscopic evidence confirms the attainment of a homogeneous reaction initiation at concentrations and conditions relevant for further crystallography studies. This was demonstrated by the oxidizing microcrystals of reduced CcO by liberation of millimolar concentrations of molecular oxygen from a caged oxygen compound. We believe this will expand the scope of available techniques for the detailed investigation of oxygen-dependant enzymes with its native substrate and facilitate further time-resolved X-ray based studies such as wide/small angle X-ray scattering and serial femtosecond crystallography.

Time-resolved serial crystallography to track the dynamics of carbon monoxide in the active site of cytochrome c oxidase

Cytochrome c oxidase (CcO) is part of the respiratory chain and contributes to the electrochemical membrane gradient in mitochondria as well as in many bacteria, as it uses the energy released in the reduction of oxygen to pump protons across an energy-transducing biological membrane. Here, we use time-resolved serial femtosecond crystallography to study the structural response of the active site upon flash photolysis of carbon monoxide (CO) from the reduced heme a3 of ba3-type CcO. In contrast with the aa3-type enzyme, our data show how CO is stabilized on CuB through interactions with a transiently ordered water molecule. These results offer a structural explanation for the extended lifetime of the CuB-CO complex in ba3-type CcO and, by extension, the extremely high oxygen affinity of the enzyme.

A simple goniometer-compatible flow cell for serial synchrotron X-ray crystallography

Serial femtosecond crystallography was initially developed for room-temperature X-ray diffraction studies of macromolecules at X-ray free electron lasers. When combined with tools that initiate biological reactions within microcrystals, time-resolved serial crystallography allows the study of structural changes that occur during an enzyme catalytic reaction. Serial synchrotron X-ray crystallography (SSX), which extends serial crystallography methods to synchrotron radiation sources, is expanding the scientific community using serial diffraction methods. This report presents a simple flow cell that can be used to deliver microcrystals across an X-ray beam during SSX studies. This device consists of an X-ray transparent glass capillary mounted on a goniometer-compatible 3D-printed support and is connected to a syringe pump via lightweight tubing. This flow cell is easily mounted and aligned, and it is disposable so can be rapidly replaced when blocked. This system was demonstrated by collecting SSX data at MAX IV Laboratory from microcrystals of the integral membrane protein cytochrome c oxidase from Thermus thermophilus, from which an X-ray structure was determined to 2.12 Å resolution. This simple SSX platform may help to lower entry barriers for non-expert users of SSX.

Simultaneous enzyme overlay membrane (EOM)-based in situ zymography and immunofluorescence technique reveals cathepsin B-like activity in a subset of tumour vessels

The analysis of the invasion front of oral squamous cell carcinoma (OSCC) has revealed a fundamental invasion-associated remodelling of the extracellular matrix as the result of a complex regulated interplay of matrix synthesis and matrix degradation. Cysteine proteinases, in particular cathepsin B, are implicated in tumour invasion in vivo and in vitro and are thought to be important mediators of metastasis. An in situ zymographic assay based on enzyme overlay membranes (EOMs) was established to define the tissue localisation of cathepsin B activity in OSCC. Using confocal laser scanning microscopy we present a double-labelling method for the rapid and reproducible simultaneous detection of cathepsin B-like activity and cellular or extracellular antigens based on an EOM and immunofluorescence technique on frozen sections. Applying this method, cathepsin B-like activity was mainly found in vascular structures within the invasive front of OSCC. Therefore, the results suggest a particular pathogenic role of cathepsin B in tumour angioneogenesis. The method can simply be transferred to other enzymes and can be recommended for more extensive studies of proteolytic activity in human malignancies.

Waveform generator for electrical impedance tomography (EIT) using linear interpolation with multiplying D/A converters

This paper describes the implementation of a multiplying digital-to-analogue (D/A) converter as a programmable waveform generator to be used in an electrical impedance tomography (EIT) system. Different digital techniques of generation waveforms are considered and the advantages and disadvantages of the chosen slope-plus-pedestal technique are presented. A wire-wrapped prototype system has been designed, and from measured frequency spectra the total harmonic distortion can be calculated to values between 1.1 and 2.9% depending on frequency.