The NADH recycling enzymes TsaC and TsaD regenerate reducing equivalents for Rieske oxygenase chemistry.

Many microorganisms use both biological and nonbiological molecules as sources of carbon and energy. This resourcefulness means that some microorganisms have mechanisms to assimilate pollutants found in the environment. One such organism is Comamonas testosteroni, which metabolizes 4-methylbenzenesulfonate and 4-methylbenzoate using the TsaMBCD pathway. TsaM is a Rieske oxygenase, which in concert with the reductase TsaB consumes a molar equivalent of NADH. Following this step, the annotated short-chain dehydrogenase/reductase and aldehyde dehydrogenase enzymes TsaC and TsaD each regenerate a molar equivalent of NADH. This co-occurrence ameliorates the need for stoichiometric addition of reducing equivalents and thus represents an attractive strategy for integration of Rieske oxygenase chemistry into biocatalytic applications. Therefore, in this work, to overcome the lack of information regarding NADH recycling enzymes that function in partnership with Rieske non-heme iron oxygenases (Rieske oxygenases), we solved the X-ray crystal structure of TsaC to a resolution of 2.18 Å. Using this structure, a series of substrate analog and protein variant combination reactions, and differential scanning fluorimetry experiments, we identified active site features involved in binding NAD+ and controlling substrate specificity. Further in vitro enzyme cascade experiments demonstrated the efficient TsaC- and TsaD-mediated regeneration of NADH to support Rieske oxygenase chemistry. Finally, through in-depth bioinformatic analyses, we illustrate the widespread co-occurrence of Rieske oxygenases with TsaC-like enzymes. This work thus demonstrates the utility of these NADH recycling enzymes and identifies a library of short-chain dehydrogenase/reductase enzyme prospects that can be used in Rieske oxygenase pathways for in situ regeneration of NADH.

Purification and characterization of a Rieske oxygenase and its NADH-regenerating partner proteins.

The Rieske non-heme iron oxygenases (Rieske oxygenases) comprise a class of metalloenzymes that are involved in the biosynthesis of complex natural products and the biodegradation of aromatic pollutants. Despite this desirable catalytic repertoire, industrial implementation of Rieske oxygenases has been hindered by the multicomponent nature of these enzymes and their requirement for expensive reducing equivalents in the form of a reduced nicotinamide adenine dinucleotide cosubstrate (NAD(P)H). Fortunately, however, some Rieske oxygenases co-occur with accessory proteins, that through a downstream reaction, recycle the needed NAD(P)H for catalysis. As these pathways and accessory proteins are attractive for bioremediation applications and enzyme engineering campaigns, herein, we describe methods for assembling Rieske oxygenase pathways in vitro. Further, using the TsaMBCD pathway as a model system, in this chapter, we provide enzymatic, spectroscopic, and crystallographic methods that can be adapted to explore both Rieske oxygenases and their co-occurring accessory proteins.

Investigation of the Effect of Turn Residues on Tetrapeptide Aldol Catalysts with ß-Turn Propensity.

Peptide catalysts for a wide diversity of reaction types contain a common motif-residues that bias the sequence toward ß-turn secondary structure. In this work, we explore what role that secondary structure plays in the catalysis of aldol reactions for primary amine tetrapeptide aldol catalysts. Using a lead tetrapeptide ß-turn catalytic sequence, we varied the i + 1 and i + 2 residues to amino acids that would affect the ß-turn propensity. We then studied the correlation between secondary structure, aldol rate enhancement, and stereoselectivity of the reaction between hydroxyacetone and 4-nitrobenzaldehyde. Using the i + 3 amide chemical shift as a measure of ß-turn character, we found a rough correlation between the peptide structure and reaction kinetics but minimal effect on stereoselectivity. These trends may help aid the design of future catalytic sequences.

The role of ß-hairpin conformation in ester hydrolysis peptide catalysts based on a TrpZip scaffold.

To explore the role of peptide conformation on catalytic activity in the context of ester hydrolysis catalysts, pairs of sequences were designed that contained or lacked ß-hairpin character. For the hydrolysis of para-nitrophenylacetate in aqueous media, we found small but consistent trends wherein His-containing sequences based on a TrpZip scaffold showed higher catalytic activity without ß-hairpin character.