Why does Nature use modular enzyme architectures for biological catalysis?

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Redox proteins, including metalloproteins, form a large portion of the protein kingdom. Metalloproteins themselves form ~ 30% of a genome. These contain metal ions either as a single atom or as part of a cluster and play a variety of life sustaining roles in the microbial, plant and animal kingdoms. Many enzymes exploit the oxidation states of metals to perform redox cycling. Fundamental biological processes in which metalloproteins participate include electron storage and transfer, dioxygen binding, storage and activation, and substrate transport, and catalysis. In many metalloenzymes such as cytochrome c oxidase (essential for mammalian life through respiratory requirements), nitrogenases and nitrite reductases (essential in view of their central position in the nitrogen cycle), hydrogenases (producers of molecular hydrogen - a candidate for a future alternative energy source), catalysis involves the controlled delivery of electrons and protons to the active site where substrate is utilised. Nitrite reductases are central to the denitrification process, an important branch of microbial bioenergetics and crucial to terrestrial and oceanic nitrogen cycling, since it makes an increasing contribution to global warming by release of N2O, an ozone-depleting and greenhouse gas some 300-fold more potent than CO2. The current proposal builds on close collaboration between the applicants where they collectively have made major contributions in the field of denitrification and have provided significant advances in our understanding of complex processes that are involved in biological mechanisms of metalloenzymes. Our combined approaches puts us in a very strong position to undertake an integrated structural-mechanistic programme that is aimed at addressing the question of whether Nature exploits tethered domains to enhance catalysis compared to transient protein complexes in biological reactions in globally important biological systems. We focus on Cu-containing nitrite reductases (CuNiRs) - exploiting their natural encounter (freely diffusing) and tethered complexes - to learn Nature's design rules for construction of optimally configured and integrated redox devices. We will elucidate design principles that define catalytic efficiencies and enable coupling of long-range electron movements to active site redox chemistry. This requires understanding of how coordinated protein movements impact on (i) mechanisms of long-range electron transfers, (ii) localised chemical change (bond formation / breakage) and (iii) how these can change the rate-limiting step in catalysis by driving the formation of different oxidation states of the active site. General design principles will emerge that will guide predictive engineering of biological redox devices for synthetic biology.New methods and approaches developed in this programme (e.g. (i) combined stopped-flow and FRET-based approach enabling the reporting on redox chemistry via a 'molecular beacon' approach and (ii) development of laboratory-based size-exclusion chromatography-small angle X-ray scattering with dynamic light scattering (SEC-SAXS-DLS) for studying protein complexes) will have broad relevance to our capabilities for studying protein complexes. These new capabilities and the scientific outcome will have significant impact on structural-mechanistic biology and keep the UK at the forefront of global effort in this important field.