An objective criterion to evaluate sequence-similarity networks helps in dividing the protein family sequence space

CANDy: Automated analysis of domain architectures in carbohydrate-active enzymes

Carbohydrate-active enzymes (CAZymes) can be found in all domains of life and play a crucial role in metabolic and physiological processes. CAZymes often possess a modular structure, comprising not only catalytic domains but also associated domains such as carbohydrate-binding modules (CBMs) and linker domains. By exploring the modular diversity of CAZy families, catalysts with novel properties can be discovered and further insight in their biological functions and evolutionary relationships can be obtained. Here we present the carbohydrate-active enzyme domain analysis tool (CANDy), an assembly of several novel scripts, tools and databases that allows users to analyze the domain architecture of all protein sequences in a given CAZy family. CANDy's usability is shown on glycoside hydrolase family 48, a small yet underexplored family containing multi-domain enzymes. Our analysis reveals the existence of 35 distinct domain assemblies, including eight known architectures, with the remaining assemblies awaiting characterization. Moreover, we substantiate the occurrence of horizontal gene transfer from prokaryotes to insect orthologs and provide evidence for the subsequent removal of auxiliary domains, likely through a gene fission event. CANDy is available at https://github.com/PyEED/CANDy.

Widespread Family of NAD+-Dependent Sulfoquinovosidases at the Gateway to Sulfoquinovose Catabolism

The sulfosugar sulfoquinovose (SQ) is produced by photosynthetic plants, algae, and cyanobacteria on a scale of 10 billion tons per annum. Its degradation, which is essential to allow cycling of its constituent carbon and sulfur, involves specialized glycosidases termed sulfoquinovosidases (SQases), which release SQ from sulfolipid glycoconjugates, so SQ can enter catabolism pathways. However, many SQ catabolic gene clusters lack a gene encoding a classical SQase. Here, we report the discovery of a new family of SQases that use an atypical oxidoreductive mechanism involving NAD+ as a catalytic cofactor. Three-dimensional X-ray structures of complexes with SQ and NAD+ provide insight into the catalytic mechanism, which involves transient oxidation at C3. Bioinformatic survey reveals this new family of NAD+-dependent SQases occurs within sulfoglycolytic and sulfolytic gene clusters that lack classical SQases and is distributed widely including within Roseobacter clade bacteria, suggesting an important contribution to marine sulfur cycling.