Mutational Analysis of Linalool Dehydratase Isomerase Suggests That Alcohol and Alkene Transformations Are Catalyzed Using Noncovalent Mechanisms

Double-duty isomerases: a case study of isomerization-coupled enzymatic catalysis

Enzymes can usually be unambiguously assigned to one of seven classes specifying the basic chemistry of their catalyzed reactions. Less frequently, two or more reaction classes are catalyzed by a single enzyme within one active site. Two examples are an isomerohydrolase and an isomero-oxygenase that catalyze isomerization-coupled reactions crucial for production of vision-supporting 11-cis-retinoids. In these enzymes, isomerization is obligately paired and mechanistically intertwined with a second reaction class. A handful of other enzymes carrying out similarly coupled isomerization reactions have been described, some of which have been subjected to detailed structure-function analyses. Herein we review these rarefied enzymes, focusing on the mechanistic and structural basis of their reaction coupling with the goal of revealing catalytic commonalities.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

Efficient Myrcene Production Using Linalool Dehydratase Isomerase and Rational Biochemical Process in Escherichia coli

Microbial synthesis of plant-based myrcene is of great interest because of its high demand, however, achieving high biosynthetic titers remains a great challenge. Previous strategies adopted for microbial myrcene production have relied on the recruitment of a multi-step biosynthetic pathway which requires complex metabolic regulation or high activity of myrcene synthase, hindering its application. Here, we present an effective one-step biotransformation system for myrcene biosynthesis from geraniol, using a linalool dehydratase isomerase (LDI) to overcome these limitations. The truncated LDI possesses nominal activity that catalyzes the isomerization of geraniol to linalool and the subsequent dehydration to myrcene in anaerobic environment. In order to improve the robustness of engineered strains for the efficient conversion of geraniol to myrcene, rational enzyme modification and a series of biochemical process engineering were employed to maintain and improve the anaerobic catalytic activity of LDI. Finally, by introducing the optimized myrcene biosynthetic capability in the existing geraniol-production strain, we achieve de novo biosynthesis of myrcene at 1.25g/L from glycerol during 84h aerobic-anaerobic two-stage fermentation, which is much higher than previously reported myrcene levels. This work highlights the value of dehydratase isomerase-based biocatalytic in establishing novel biosynthetic pathways and lays a reliable foundation for the microbial synthesis of myrcene.

Potential enzymes involved in beer monoterpenoids transformation: structures, functions and challenges

Monoterpenes are important flavor and fragrance compounds in food. In beer, the monoterpenes mainly come from hops added during boiling process. Biotransformations of monoterpene which occurred during fermentation conferred beer with various kinds of aroma profiles, which can be mainly attributed to the contribution of enzymes in yeast. However, there are few reports on the identification and characterization of these enzymes in yeast. Illustrating the structure and functions of key enzymes related to transformations will broaden their potential applications in beer or other foodstuffs. Monoterpenoids including terpene hydrocarbons (limonene, myrcene, and pinene) and terpene alcohol (linalool, geraniol, nerol, and citronellol) gave the beer flower-like or fruit-like aroma. The biotransformation of monoterpenes and monoterpene alcohols in bacteria and yeast, and potential enzymes related to the transformation of them are reviewed here. Enzymes primarily are dehydrogenases including linalool dehydrogenase/isomerase, geraniol/geranial dehydrogenase, nerol dehydrogenase and citronellol dehydrogenase. Most of them are substrate-specific or substrate-specific after modifications by biotechnology methods, and part of them have been expressed in E. coli, while the purification and catalytic rate is very low. Efforts should be made to acquire abundant enzymes, and to fabricate enzyme-expressing yeast, which can be further applied in beer fermentation system.highlightsMonoterpenoids contributed to the flavor of food, especially beer.Transformation of monoterpenoids occurred during fermentation.Various kinds of enzymes are involved in the transformation of monoterpenoids in bacteria, yeast, etc.Crystal structures of these enzymes have been partially resolved.Few enzymes are further applied in food system to obtain abundant flavor.