Understanding the recognition and utilization of homogalacturonan by Bacteroides thetaiotaomicron

Abstract

Homogalacturonan (HG) is a structural plant cell wall polysaccharide and a key source of dietary fiber. The human genome does not contain a single enzyme known to be involved in pectin digestion, and therefore, in order to modify HG fibers and potentially extract nutritional value, humans rely on a consortium of symbiotic intestinal bacteria, such as Bacteroides thetaiotaomicron, to deconstruct and to ferment this complex carbohydrate into host absorbable products. B. thetaiotaomicron contains over 300 predicted carbohydrate active enzymes within its genome that are primarily organized into sugar-selective metabolic pathways called Polysaccharide Utilization Loci PULs (PULs). One such PUL (PUL75: BT4108-BT4124), is activated by HG and is believed to contain enzymes that convert polymerized HG into monosaccharides (GalA). This study reports molecular biology, biochemistry, and functional genomics data that characterize the function of PUL75 gene products involved in HG utilization. Based on these findings, a model for the step-wise process of HG recognition, transport and modification by PUL75 is proposed.