Crystal Structure of the Putidaredoxin Reductase·Putidaredoxin Electron Transfer Complex

Abstract

In the camphor monooxygenase system from Pseudomonas putida, the [2Fe-2S]-containing putidaredoxin (Pdx) shuttles electrons between the NADH-dependent putidaredoxin reductase (Pdr) and cytochrome P450 cam . The mechanism of the Pdr⅐Pdx redox couple has been investigated by a variety of techniques. One of the exceptions is x-ray crystallography as the native partners associate weakly and resist co-crystallization. Here, we present the 2.6-Å x-ray structure of a catalytically active complex between Pdr and Pdx C73S/C85S chemically cross-linked via the Lys 409Pdr -Glu 72Pdx pair. In Pseudomonas putida, the camphor monooxygenase system uses NADH as a source of electrons and consists of three soluble proteins: FAD-containing putidaredoxin reductase (Pdr, 3 45.6 kDa), [2Fe-2S]-containing putidaredoxin (Pdx, 11.4 kDa), and cytochrome P450 cam (P450 cam , 46.6 kDa) (1). Pdx receives reducing equivalents from Pdr in two one-electron steps and delivers them one at a time to P450 cam . Acting as a shuttle, Pdx forms transient electron transfer (ET) complexes with its redox partners during turnover (2-6). The x-ray structures of all components of the camphor monooxygenase have been determined (7-10), but neither Pdr⅐Pdx nor Pdx⅐P450 cam native complexes have been crystallized thus far.The mechanism of interaction and interprotein ET in the Pdr⅐Pdx pair has been the focus of several research groups including ours. Significant progress has been made in the general understanding of how the Pdr⅐Pdx complex is formed and functions. In particular, both two-and one-electron reduced species of Pdr were identified as catalytically competent redox intermediates (3, 4, 11); ionic and hydrophobic interactions as well as steric complementarity were proven to contribute to molecular recognition between the partners (4, 12, 13) and involve Tyr 33 , Asp 38 , Arg 66 , Glu 72 , and Cys 73 of Pdx (14 -16); and, based on the x-ray structures of the flavo-and iron-sulfur proteins (8, 10, 17), a computer model for the Pdr⅐Pdx ET complex was generated and experimentally tested (16). However, the precise manner of the Pdr-Pdx interaction, the docking sites, and interface residues remained unknown. The most direct way to obtain this information would be determination of the x-ray structure of the Pdr⅐Pdx complex, but, despite our multiple attempts, the native proteins resisted co-crystallization. This could in part be due to weak association (K d ϭ 66 M) (16) and quick inactivation of wild type Pdx in solution (8).To overcome this problem, we attempted to produce a functionally active Pdr⅐Pdx conjugate. Having screened various cross-linking agents, reaction conditions, and Pdx mutants, we prepared a catalytically competent complex between wild type Pdr and Pdx C73S/C85S covalently linked by 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide (EDC) via the Lys 409Pdr -Glu 72Pdx salt bridge (6). The double mutant of Pdx was chosen for a functional analysis because substitution of cysteines 73 and 85 with serine, a highly isosteric analogue, only mildly aff...

Molecular Characterization of a Class I P450 Electron Transfer System from Novosphingobium aromaticivorans DSM12444

“…This is supported by a computer model of the PdR-Pdx complex (52), mutagenesis studies of the PuR/Pux interaction (14), the crystal structures of BphA4-BphA3 complexes (51), and the crystal structures of cross-linked PdR-Pdx and AdR-Adx (28,53). The si side surfaces of ONFRs structurally characterized to date have largely positive electrostatic potential surrounding a central patch of neutral residues located directly above the FAD isoalloxazine ring (Fig.…”

“…KKS102, have been reported (28,51). Recently, the structure of a covalently cross-linked PdR-Pdx complex (via Lys 409 and Glu 72 ) has been solved, and the resultant investigations indicated that there is a high degree of fine tuning in the protein/ protein interactions to allow efficient electron transfer (53,71). The crystal structures of PuR and PuxB have also provided insight into the recognition interactions (14,15,29).…”

“…3a and supplemental Fig. S7, a and b (25,(51)(52)(53)(54)(55). Ionic interactions involving charged residues on the periphery of this central patch can also be a determining factor.…”

Review of NAD(P)H-dependent oxidoreductases: Properties, engineering and application

“…14). The enzymes differ on the route followed by electrons from the Fe-S cluster to FAD, with some of them involving several intermediate residue side chains [152]. Additionally, Fig.…”

Structural biology of redox partner interactions in P450cam monooxygenase: A fresh look at an old system

“…Our results suggested also that the inter-protein electron transfer could proceed via different pathways that likely include Asp38 and Cys39 but do not involve Trp106. This initial effort has been followed by successful production of a functional covalent Pdr-Pdx complex [43], whose crystal structure was determined to 2.6 Å resolution [44] and experimentally tested [45]. …”