Multifunctional Enzymes in the shikimate pathway

Abstract

Multifunctional enzymes are instances of natural fusion proteins, possessing multiple catalytic activities. Via the close proximity of their catalytic domains, they take advantage from concentration of the reactants and may feature allosteric regulation and substrate channeling effects. Substantial progress has been made regarding the kinetic mechanisms of individual enzymes, but the knowledge in the areas of multifunctional enzymes and also transient complexes is rather limited. The seven step shikimate pathway, which is a metabolic route to aromatic compounds, has a number of bifunctional, trifunctional, tetrafunctional and pentafunctional fusion proteins in different organisms. Most notable is the pentafunctional AROM complex, which comprises the central five steps of the pathway, which has withstood structural characterization for decades. In this work, we shed light on the pentafunctional AROM complex and the three bifunctional enzymes Tm_AroKB, Fp_AroEK and Mb_AroKE of the shikimate pathway by characterizing them functionally and structurally and comparing them to their monofunctional homologs. For the bifunctional proteins, Tm_AroKB and Fp_AroEK the kinetic analysis revealed that their activity is similar to that reported for their monofunctional counterparts. In addition, the kinetic activity of the truncated monofunctional Tm_AroKKB was found to be similar to Tm_AroKB. Interestingly, no allosteric regulation was observed between the two domains in Tm_AroKB. The kinetic analysis of the K domain of Mb_AroKE revealed that it is slower than the other enzymes in this study. However, the genome of the respective organism Methanoregula boonei also contains a monofunctional Mb_AroK, which might play the major role in the shikimate pathway. All crystal structures of the bifunctional enzymes show rigid inter-domain interfaces, with all the catalytic sites solvent accessible. Tm_AroKB is a dimer both in solution and in crystalline state. Remarkably, it contains an intrinsically bound NAD molecule. In all bifunctional proteins, the structure of the individual domains corresponds to that of the monofunctional homologs. However, the domain arrangement in the structures of Fp_AroEK and Mb_AroKE are different. Both arrangements bring the catalytic sites closer and could be part of potential higher order assemblies. Therefore, we hypothesized that the individual enzymes of the pathway form transient complexes for efficient substrate funneling and regulation. In silico models of such assemblies were generated based on both Fp_AroEK, and Mb_AroKE, via superimposition on the bifunctional proteins 4 Tm_AroKB and At_AroDE. These two models are not identical but they both represent a plausible, compact assembly with all active sites solvent accessible. Finally, we studied the pentafunctional AROM complex. The catalytic turnover of the AROM complex is significantly higher than that of the monofunctional E.coli enzymes. The crystal structure of AROM complex, which was found to be different from both in silico models, shows that its domains are attached to each other via rigid interfaces and that all the catalytic sites are solvent accessible. Using SAXS and XL-MS, we verified that the conformation of the AROM complex is very similar in solution and in crystalline state. In a computational approach, we further mapped the various conformational states of the individual enzymatic domains from the PDB onto the crystal structure, generating a structural ensemble representing the conformational space of the AROM complex. This ensemble reveals that the complex is optimized for spatial compatibility of the domains, allowing all necessary conformational changes during the catalytic cycle to happen without steric clashes between domains. Since the shikimate pathway is absent from mammals, it poses a classical drug target. Until now, only glyphosate is utilized as a herbicide for targeting the shikimate pathway. The insight obtained in this study suggests novel approaches to target the shikimate pathway. Most notable is the conformational space of the AROM complex, which is essential in fungi and protists. By targeting the conformational flexibility of the complex, catalytically necessary conformational transitions could be inhibited, which could result in novel, specific fungicides.