How an enzyme achieves such high rates of catalysis in comparison to its solution counterpart reaction has baffled scientists for many decades. Much of our understanding of enzyme function is derived from research devoted to enzyme chemical reactions and analysis of static three-dimensional images of individual enzyme molecules. However, more recently, a role of protein dynamics in facilitating enzyme catalysis has emerged. It is often challenging to probe how protein motions are correlated to and impact on the catalytic cycle of enzymes. Nevertheless, this subject must be addressed to further our understanding of the roots of enzyme catalysis. Herein, this research question is approached by studying the link between protein domain dynamics and electron transfer chemistry in the diflavin oxidoreductase family of enzymes. Previous studies conducted on the diflavin oxidoreductases have implied a role of protein domain dynamics in catalysing electron transfer chemistry. However, diflavin oxidoreductase motions have not been experimentally correlated with mechanistic steps in the reaction cycle. To address these shortcomings, a 'real-time' analysis of diflavin oxidoreductase domain dynamics that occur during enzyme catalysis was undertaken. The methodology involved specific labelling of diflavin oxidoreductases (cytochrome P450 reductase, CPR, and neuronal nitric oxide synthase, nNOS) with external donor-acceptor fluorophores that were further used for time-resolved stopped-flow Förster resonance energy transfer (FRET) spectroscopy measurements. This approach to study enzyme dynamics was further linked with traditional UV-visible stopped-flow approaches that probed enzymatic electron transfer chemistry. Results showed a tight coupling between the kinetics of electron transfer chemistry and domain dynamics in the two diflavin oxidoreductase systems studied. Moreover, through the use of a flavin analogue (5-deazaflavin mononucleotide) and isotopically labelled nicotinamide coenzymes (pro-S/R NADP2H), key steps in the reaction mechanism were correlated with dynamic events in calmodulin, the partner protein of nNOS.The approaches developed in this project should find wider application in related studies of complex electron-transfer enzymes. Altogether, this research emphasises the key link between protein domain motions and electron transfer chemistry and provides a framework to describe the relationship between domain dynamics and diflavin oxidoreductase function.