Racemization in Reverse: Evidence that D-Amino Acid Toxicity on Earth Is Controlled by Bacteria with Racemases

Gaosen Zhang,Henry J Sun,Vipul Bansal

doi:10.1371/journal.pone.0092101

Abstract

D-amino acids are toxic for life on Earth. Yet, they form constantly due to geochemical racemization and bacterial growth (the cell walls of which contain D-amino acids), raising the fundamental question of how they ultimately are recycled. This study provides evidence that bacteria use D-amino acids as a source of nitrogen by running enzymatic racemization in reverse. Consequently, when soils are inundated with racemic amino acids, resident bacteria consume D- as well as L-enantiomers, either simultaneously or sequentially depending on the level of their racemase activity. Bacteria thus protect life on Earth by keeping environments D-amino acid free.

Highlights

With the exception of glycine, all amino acids are chiral and may exist as levorotatory (L) or dextrorotatory (D) enantiomer
Because D-alanine is an essential component of their cell wall [6,31,32], all bacteria are expected to synthesize alanine racemases constitutively
Our study suggests that when D-alanine is available externally, alanine racemases are temporarily freed from anabolic commitment to serve a detoxification and catabolic function, turning excess Dalanine into a source of nitrogen

Summary

Introduction

With the exception of glycine, all amino acids are chiral and may exist as levorotatory (L) or dextrorotatory (D) enantiomer. In some organisms, racemization is enzymatically catalyzed (i.e. by racemases), and the resultant D-amino acids are incorporated into non-protein molecules. Such molecules include peptide antibiotics [3], siderophores [4], surfactins [5], and peptidoglycans [6]. The latter, which contain D-alanine, D-glutamic acid, and occasionally D-aspartic acid as well, are ubiquitous in bacterial and cyanobacterial cell walls. In North American grassland soils, for instance, more than 10% of alanine, glutamic acid, aspartic acid, and leucine exist in D-forms [18]

Methods

Results

Conclusion