Catabolism of Unnatural Carbohydrates by Micro-Organisms

Abstract

Publisher Summary The chapter focuses on the extensive investigations into the metabolism of unnatural carbohydrates by microorganism. A number of microbial cultures for the ability to ferment rare five-carbon sugars are tested. In tests with fermentation media, this strain was able to metabolize all of the eight aldopentose structures (D- and L-ribose, D- and L-arabinose, D- and L-xylose and D- and L-lyxose) and all of the four pentitol structures (ribitol, D- and L-arabitol, and xylitol). This seemed remarkable because only three of the aldopentoses (D-ribose, L-arabinose and D-xylose) and only two of the pentitols (ribitol and D-arabitol) were considered to be available as growth substrates in nature. The versatility of this organism in its ability to utilize these sugars as sources of carbon and energy is discussed in this chapter. The possible exception was L-ribose, which was not available for testing as a potential growth substrate. It has been recognized in a number of laboratories that studies on the mechanisms by which micro-organisms utilize novel substrates may lead to increased understanding of the events, which result in the evolution of the catabolic pathways in microorganisms. In spite of the facts that L-arabinose and L-arabitol are degraded through this common intermediate and that ribitol dehydrogenase is required to catalyse the first step in degradation of L-arabitol, cells constitutive for ribitol dehydrogenase maintain high levels of the enzyme during growth on L-arabitol. The relatively poor activity of the enzyme for oxidation of L-arabitol as a substrate results in slow metabolism of L-arabitol and a decreased accumulation of the metabolic intermediates and high-energy compounds, which could lead to repression of the ribitol pathway enzymes.