Increasing the Thermostable Sugar-1-Phosphate Nucleotidylyltransferase Activities of the Archaeal ST0452 Protein through Site Saturation Mutagenesis of the 97th Amino Acid Position
Authors: Honda Yuki , Zang Qian , Shimizu Yasuhiro , Dadashipour Mohammad , Zhang Zilian , Kawarabayasi Yutaka 
Published: November 2016

The ST0452 protein is a bifunctional protein exhibiting sugar-1-phosphate nucleotidylyltransferase (sugar-1-P NTase) and amino-sugar-1-phosphate acetyltransferase activities and was isolated from the thermophilic archaeon
Sulfolobus tokodaii
. Based on the previous observation that five single mutations increased ST0452 sugar-1-P NTase activity, nine double-mutant ST0452 proteins were generated with the intent of obtaining enzymes exhibiting a further increase in catalysis, but all showed less than 15% of the wild-type
N
-acetyl-
d
-glucosamine-1-phosphate uridyltransferase (GlcNAc-1-P UTase) activity. The Y97A mutant exhibited the highest activity of the single-mutant proteins, and thus site saturation mutagenesis of the 97th position (Tyr) was conducted. Six mutants showed both increased GlcNAc-1-P UTase and glucose-1-phosphate uridyltransferase activities, eight mutants showed only enhanced GlcNAc-1-P UTase activity, and six exhibited higher GlcNAc-1-P UTase activity than that of the Y97A mutant. Kinetic analyses of three typical mutants indicated that the increase in sugar-1-P NTase activity was mainly due to an increase in the apparent
k
cat
value. We hypothesized that changing the 97th position (Tyr) to a smaller amino acid with similar electronic properties would increase activity, and thus the Tyr at the corresponding 103rd position of the
Escherichia coli
GlmU (
Ec
GlmU) enzyme was replaced with the same residues. The Y103N mutant
Ec
GlmU showed increased GlcNAc-1-P UTase activity, revealing that the Tyr at the 97th position of the ST0452 protein (103rd position in
Ec
GlmU) plays an important role in catalysis. The present results provide useful information regarding how to improve the activity of natural enzymes and how to generate powerful enzymes for the industrial production of sugar nucleotides.

IMPORTANCE
It is typically difficult to increase enzymatic activity by introducing substitutions into a natural enzyme. However, it was previously found that the ST0452 protein, a thermostable enzyme from the thermophilic archaeon
Sulfolobus tokodaii
, exhibited increased activity following single amino acid substitutions of Ala. In this study, ST0452 proteins exhibiting a further increase in activity were created using a site saturation mutagenesis strategy at the 97th position. Kinetic analyses showed that the increased activities of the mutant proteins were principally due to increased apparent
k
cat
values. These mutant proteins might suggest clues regarding the mechanism underlying the reaction process and provide very important information for the design of synthetic improved enzymes, and they can be used as powerful biocatalysts for the production of sugar nucleotide molecules. Moreover, this work generated useful proteins for three-dimensional structural analysis clarifying the processes underlying the regulation and mechanism of enzymatic activity.