, 1963) It consists of two domains; a hydroxylase N-terminal dom

, 1963). It consists of two domains; a hydroxylase N-terminal domain with one molecule of noncovalently bound PQQ and Ca2+ at its active site as cofactors and a cytochrome c C-terminal binding domain with one covalently bound molecule of c-type haem which acts as an electron acceptor following lupanine dehydration (Hopper et al., 2002). Periplasmic targeting of the recombinant LH enzyme in Escherichia coli requires the co-expression of cytochrome c maturation factors and complex post-translational modifications that include signal peptide processing, covalent haem attachment to the C-terminal cytochrome c domain and putative disulphide bond formation

PD0325901 nmr (Stampolidis et al., 2009). Sequence analysis

with Clustal W (Larkin et al., 2007) reveals many common features of LH to members of the quinohaemoprotein family such as methanol dehydrogenase from Methylobacterium extorquens and particularly, ethanol dehydrogenase (EDH) from Comamonas testosteroni (Fig. 1). Some of the highly conserved residues among quinohaemoproteins involved in PQQ binding and at the active site of the enzyme are present Selleckchem Ku 0059436 in LH as is the invariant amino acid, Trp, which forms the floor of the active site cavity (Anthony, 1996; Hopper & Kaderbhai, 2003). In quinohaemoproteins, PQQ is usually sandwiched between a disulphide bond formed by two neighbouring Cys (Chen et al., 2002), for example, in methanol dehydrogenase 103,104Cys (Afolabi et al., 2001) and ethanol dehydrogenase 116,117Cys (Mennenga et al., 2009). The role of this bond is still a mystery. One hypothesis is that the disulphide bridge could potentially serve as an intraprotein redox centre, acting as a functional switch by relaying electrons from PQQ to the terminal acceptor in a similar manner to ferredoxin:thioredoxin reductase (Dai et al., 2000), glutathione reductase and lipoamide

dehydrogenase (White et al., 1993). A second theory claims that the bond could have a structural role for proper positioning of PQQ within the active site of the enzyme (Oubrie et al., 2002). However, LH possesses, in total, four Cys residues, two are part of the cytochrome c motif (586Cys and 589Cys), and the remaining two are separated by Methamphetamine 18 amino acids (124Cys and 143Cys). In this study, we attempted to establish the presence of the disulphide bond using chemical means and role in recombinant LH using site-directed mutagenesis with 143CysSer and 124,143CysSer mutations in E. coli. All chemicals were purchased from Sigma, Qiagen Ni-NTA agarose from Qiagen, and electrophoresis reagents were obtained from Bio-Rad and BDH (UK). Restriction enzymes and DNA-modifying enzymes were purchased from New England Biolabs and Promega (UK). Escherichia coli TB1 and pINK-LH-His4 construct were from Dr M. A. Kaderbhai Laboratory.

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