Controlling the Regiospecific Oxidation of Aromatics via Active Site Engineering of Toluene para-Monooxygenase of Ralstonia pickettii PKO1

Abstract

A primary goal of protein engineering is to control catalytic activity. Here we show that through mutagenesis of three active site residues, the catalytic activity of a multicomponent monooxygenase is altered so that it hydroxylates all three positions of toluene as well as both positions of naphthalene. Hence, for the first time, an enzyme has been engineered so that its regiospecific oxidation of a substrate can be controlled. Through the A107G mutation in the ␣-subunit of toluene para-monooxygenase, a variant was formed that hydroxylated toluene primarily at the ortho-position while converting naphthalene to 1-naphthol. Conversely, the A107T variant produced >98% p-cresol and p-nitrophenol from toluene and nitrobenzene, respectively, as well as produced 2-naphthol from naphthalene. The mutation I100S/G103S produced a toluene para-monooxygenase variant that formed 75% m-cresol from toluene and 100% m-nitrophenol from nitrobenzene; thus, for the first time a true meta-hydroxylating toluene monooxygenase was created.Enzymes are attractive catalysts for chemical synthesis because of their exceptional enantio-and regioselectivities; hence, enzymes can be used in both simple and complex syntheses without the need for the blocking and deblocking steps often found in their organic counterparts (1, 2). The number of biocatalytic processes that are being performed on a large scale is rapidly increasing, and it is projected that this growth will continue (3, 4) to become 30% of the chemical business by the year 2050 (5). Although the majority of currently used industrial enzymes are hydrolases (4), there is growing interest in the application of oxygenases (5). Oxidative biotransformations use oxygen as an inexpensive, environmentally friendly oxidant in contrast to toxic chemical oxidants, and they exceed their chemical equivalent in regiospecificity and enantioselectivity (6, 7). Among bacterial oxygenases, toluene monooxygenases have a lot of potential for bioremediation and chemical synthesis, as we have used mutagenesis to make, among other things, variants that remediate one of the world's worst pollutants (trichloroethylene) (8), form a rainbow of colors from indole with a single enzyme, and make the anti-cancer compound indirubin (9). These variants also make useful compounds like 1-naphthol (15,000 ton/year market) (8), nitrohydroquinone (precursor for therapeutics for Alzheimer and Parkinson disease) (10), and 4-nitrocatechol (inhibitor of nitric-oxide synthase) (11). For these industrial applications, control of regiospecific oxidation of aromatics is of vital importance. Further, regiospecific oxidation of toluene, substituted benzenes, and naphthalene by these monooxygenases presents an important challenge both in terms of the three distinct regiospecific oxidations possible for the initial hydroxylation of toluene as well as in terms of the usefulness of both the mono-and dihydroxylated products.Toluene para-monooxygenase (TpMO, 1 formerly known as T3MO) of Ralstonia pickettii PKO1 is a soluble, non-heme, ...

Molecular Determinants of the Regioselectivity of Toluene/ o -Xylene Monooxygenase from Pseudomonas sp. Strain OX1

“…The different regioselectivities shown by BMMs have been attributed previously to differences in the shape of the active site pocket, an idea supported by the fact that several point mutations in the active sites of T4MO, toluene p-monooxygenase, ToMO, MMO, and B. cepacia G4 T2MO cause large variations in the regioselectivities of these BMMs (2,7,26,30,34,35). In a previous paper (4), we have reported that mutations at position 103 of the A subunit change the regioselectivity of ToMO from Pseudomonas sp.…”

Atomic Picture of Ligand Migration in Toluene 4-Monooxygenase

“…The specific activity (nmol/min/mg protein) was calculated as the ratio of the initial transformation rate and the total protein content, 0.24 [mg protein/ml/OD 600nm ]. 10,30,31 Activity data reported in this paper are based on at least three independent results. Analytical methods were described previously.…”

“…In contrast to other cases, this residue is located 30Å away from the active site, near the interface of subunits  and , suggesting that its catalytic influence should be the result of a change in the active site dynamics or in the ligand delivery. Several crystallographic studies of the BMM superfamily, proposed a common channel through the  subunit connecting the diiron center to the surface; this pathway involves surface residues D285 and E214 in T4MO 9,10 . Additional reports described two other hydrophobic cavities, one near the active site pocket and another which is located near the interface of the  and  subunits 1,5,10 .…”

“…Several crystallographic studies of the BMM superfamily, proposed a common channel through the  subunit connecting the diiron center to the surface; this pathway involves surface residues D285 and E214 in T4MO 9,10 . Additional reports described two other hydrophobic cavities, one near the active site pocket and another which is located near the interface of the  and  subunits 1,5,10 . Measurements of oxygen migration by the formation rates of a peroxodiiron(III) intermediate, suggested that the two cavities may connect via movement of the protein components thus regulating oxidation rate 10 .…”

“…Additional reports described two other hydrophobic cavities, one near the active site pocket and another which is located near the interface of the  and  subunits 1,5,10 . Measurements of oxygen migration by the formation rates of a peroxodiiron(III) intermediate, suggested that the two cavities may connect via movement of the protein components thus regulating oxidation rate 10 . This was further supported by the findings describing changes in the volume of the pockets upon binding of the effector protein to the hydroxylase, thus restricting free access from the solvent to the active site 1,5 .…”