The transition metals tungsten and molybdenum are the heaviest metals found in biological systems and are embedded in the cofactor of several metalloenzymes. As a result of their redox activity, they provide great catalytic power in these enzymes and facilitate chemical reactions that would not occur using only the functionalities of natural amino acids. For their functionality these enzymes depend on a metal cofactor, which consists of at least one metal binding pterin (MPT) and a tungsten or molybdenum ion, but the complete make-up of the cofactor differs per enzyme group. One of these enzyme groups comprises the AOR-family enzymes. These enzymes have the ability to oxidize a range of aldehyde substrates into their corresponding carboxylic acid products. Next to this, they are also the only known catalysts able to perform the thermodynamically challenging reduction reaction of carboxylic acids to aldehydes. These enzymes are currently obtained by purification from the hyperthermophilic archaeon Pyrococcus furiosus. This process, however, does not yield a large amount of enzyme, since it is naturally expressed at moderate levels. For that reason, other production methods need to be considered if the enzyme is to be used on a large scale. These alternatives include the use of a recombinant expression system. The recombinant expression of W-dependent enzymes in different host organisms, such as Escherichia coli, has already been attempted for different enzymes, but with varying success. This shows that more research on the production, and especially incorporation of the metal cofactor, is necessary to achieve a successful production and use of recombinant AOR-family enzymes.