The Structure of the Active Site H-Cluster of [FeFe] Hydrogenase from the Green Alga Chlamydomonas reinhardtii Studied by X-ray Absorption Spectroscopy

The [FeFe] hydrogenase (CrHydA1) of the green alga Chlamydomonas reinhardtii is the smallest hydrogenase known and can be considered as a "minimal unit" for biological H(2) production. Due to the absence of additional FeS clusters as found in bacterial [FeFe] hydrogenases, it was possible to specifically study its catalytic iron-sulfur cluster (H-cluster) by X-ray absorption spectroscopy (XAS) at the Fe K-edge. The XAS analysis revealed that the CrHydA1 H-cluster consists of a [4Fe4S] cluster and a diiron site, 2Fe(H), which both are similar to their crystallographically characterized bacterial counterparts. Determination of the individual Fe-Fe distances in the [4Fe4S] cluster ( approximately 2.7 A) and in the 2Fe(H) unit ( approximately 2.5 A) was achieved. Fe-C( horizontal lineO/N) and Fe-S bond lengths were in good agreement with crystallographic data on bacterial enzymes. The loss of Fe-Fe distances in protein purified under mildly oxidizing conditions indicated partial degradation of the H-cluster. Bond length alterations detected after incubation of CrHydA1 with CO and H(2) were related to structural and oxidation state changes at the catalytic Fe atoms, e.g., to the binding of an exogenous CO at 2Fe(H) in CO-inhibited enzyme. Our XAS studies pave the way for the monitoring of atomic level structural changes at the H-cluster during H(2) catalysis.