Identification of two highly promiscuous thermostable sugar nucleotidylyltransferases for glycorandomization.

Glycorandomization is a process that improves the efficacy of glycoconjugates
by the addition of a diverse array of sugars to secondary metabolites
and antibiotics of pharmaceutical importance. This process, which employs
sugar nucleotidylyltransferases (SNTs) and glycosyl transferases (GTs) in
tandem, would benefit by the employment of promiscuous enzymes, i.e.
those with the ability to utilize diverse noncanonical substrates. As promiscuous
GTs are available, here we set out to identify promiscuous SNTs. For
this, we began with a detailed family-wide characterization of SNTs. Earlier,
we had proposed that SNTs could be classified into two major groups – I
and II. They share a common structural framework and utilize a similar catalytic
mechanism. Subtle variations in the way two magnesium ions —
Mg2þ
A and Mg2þ
B — are stabilized by metal ion coordination motifs led to
their classification into diverse subgroups viz. I-A, I-B, I-C, II-A, and II-B.
Based on this classification, here we investigate promiscuity across the entire
family of SNTs. We study the utilization of several sugar phosphates and
nucleotides by the various subgroups of SNTs to understand substrate specificity
and promiscuity in these. We find that promiscuity is prevalent among
SNTs; and in particular, in the thermophilic homologs. In principle, promiscuity
profiling identified four new SNTs that can be employed for the production
of sugar-nucleotide libraries. However, assaying for their ability to
simultaneously utilize multiple substrates in a single-pot reaction, we find
two thermophilic SNTs- TMGA, an adenylyltransferase from Thermotoga
maritima and PHGT, a thymidylyltransferase from Pyrococcus
horikoshii that are readily employable for the production of diverse
sugar-nucleotides.