The enzyme POR (protochlorophyllide oxidoreductase), from the family of alcohol dehydrogenases, reduces protochlorophyllide into chlorophyllide on the absorption of light. The reduction involves the transfer of two protons and two electrons and is an important regulatory step in the biosynthesis of chlorophyll. In recent years, due to the availability of large quantities of the pure enzyme, much of the catalytic reaction has been unravelled by using a variety of spectroscopic methods, including ultrafast initial events in catalysis. In addition, it has been demonstrated that a light-activated conformational change of the protein is necessary to activate catalysis. This makes POR a very important model system to study the relationship between structural changes of enzymes and functionality. © The Authors Journal compilation © 2009 Biochemical Society.