Structural and Biochemical Studies of the Initiation Steps in Three Natural Product Biosynthetic Pathways

Abstract

Natural products are a rich source of diverse chemical compounds, many with pharmaceutical potential. Structural and biochemical investigations into the initiation steps of three natural product biosynthetic pathways were conducted. These findings contribute to the adaptation of natural products for industrial applications or for improved pharmaceutical properties as well as the discovery of novel enzymatic chemistry.

The olefin synthase from the marine cyanobacterium Synechococcus sp. PCC7002 converts stearic acid to 1-nonadecene, a terminal olefin. Biologically generated length-controlled olefins have potential applications as lubricants, surfactants, chemical feedstocks, and biofuels. The fatty acid-ACP ligase (FAAL) domain of olefin synthase selects stearic acid for conversion to 1-nonadecene, dictating the length of olefin produced. Crystal structures of the SynFAAL guided the creation of a number of variants that led to the discovery of an electrostatic fatty acid selection mechanism and a better understanding of the basis of fatty acid length selectivity.

The cahuitamycin pathway of Streptomyces gandocaensis natively produces three cahuitamycin variants, which are all potent inhibitors of Acinetobacter baumannii biofilm formation. Variation originates in the starter unit selected by aryl-AMP ligase CahJ. Crystal structures of several CahJ-substrate complexes provided a structure-based explanation of substrate selectivity in the creation of natural cahuitamycins. Together with a comprehensive set of substrate selectivity data, the structure provides a roadmap for the creation of novel cahuitamycin congeners for future investigation as biofilm inhibitors.

Apratoxin A from the marine cyanobacterium Moorea bouillonii contains a rare t-butyl group. The steps for t-butyl biosynthesis likely involve novel enzymatic reactions by domains within AprA, the first protein of the biosynthetic pathway. A di-domain of AprA containing a type I S-adenosylmethionine dependent methyltransferase (MT) domain was shown to possess both acyltransfer and methylation activity. The crystal structure of the di-domain indicated that the MT domain performs both reactions and that the other domain is vestigial.

The initiation steps are an important source of chemical diversity in natural product biosynthesis. Understanding the mechanisms of substrate recognition used by initiation enzymes could allow for the rational redesign of substrate specificity leading to biological products with improved characteristics.