Recently, a novel Fe-hydrogenase from a high rate of hydrogen producing Enterobacter cloacae strain IIT-BT08 was identified and partially characterized. This 147 residue protein was found to be much smaller than previously known Fe-hydrogenases, yet retaining a high catalytic activity. We predicted the structure of this protein and found it to be structurally similar to one of the two sub-domains containing the catalytic H-cluster so far jointly present in all other Fe-hydrogenases. This novel architecture allows a tentative explanation of protein function with the high rate of catalytic activity being due to it missing regulatory sub-domain, presumably allowing higher enzymatic activity at the cost of greater exposure to oxygen inactivation. This new insight may improve our understanding of the molecular and functional organization of other, more complex Fe-hydrogenases. (c) 2005 Elsevier Inc. All rights reserved.