Three primary classes/groups of [FeFe]-hydrogenases (Hyd) were identified.

Variation in F-, C- and H-cluster domains of Hyd did not vary by group classification.

Hyd that bifurcate are multimeric and belong to two of the three classes.

Regulatory proteins encoded in multimeric Hyd gene neighborhoods were identified.

Mass spectrometry of multimeric Hyd indicates post-translational modification.