An Engineered Cytidine Deaminase for Biocatalytic Production of a Key Intermediate of the Covid-19 Antiviral Molnupiravir

The Covid-19 pandemic highlights the urgent need for cost-effective processes to rapidly manufacture antiviral drugs at scale. Here we report a concise biocatalytic process for Molnupiravir, a nucleoside analogue recently approved as an orally available treatment for SARS-CoV-2. Key to the success of this process was the development of an efficient biocatalyst for the production of N-hydroxy-cytidine through evolutionary adaption of the hydrolytic enzyme cytidine deaminase. This engineered biocatalyst performs >85 000 turnovers in less than 3 h, operates at 180 g/L substrate loading, and benefits from in situ crystallization of the N-hydroxy-cytidine product (85% yield), which can be converted to Molnupiravir by a selective 5'-acylation using Novozym 435.

A 3-deazauracil-resistant mutant of Bacillus subtilis with increased production of cytidine

Bacillus subtilis No. 344 is a cytidine-producing mutant strain derived from wild type strain No. 122. When 3-deazauracil-resistant mutants were derived from strain No. 344, some of the mutants had higher productivities of cytidine. Among them, strain No. 428 accumulated 14.2 mg/ml cytidine in the culture. Cytidine 5'-triphosphate (CTP) synthetase from strain No. 428 changed to be free from feedback inhibition by CTP, compared with the enzyme from strain No. 344.

Incorporation of 5-fluorouracil into RNA by Mycobacterium avium complex strain LM1

The incorporation of 5-fluorouracil (FUra) into RNA of Mycobacterium avium complex strain LM1 was evaluated. Cells were labeled with either [14C]FUra or [3H]uracil and the ribonucleosides were analyzed by high-performance liquid chromatography. The identification of the ribonucleosides was facilitated by the use of an isocratic system that provided unambiguous identification of the RNA pyrimidine components. Uracil was incorporated into RNA as uridine, but an equal amount was converted to cytidine. [14C]FUra was incorporated directly into RNA as 5-fluorouridine and there was no evidence of its conversion to other pyrimidines. The ratio of 5-fluorouridine:uridine was 2.8-fold greater for cells grown in 100 micrograms FUra/mL than for cells grown in 20 micrograms FUra/mL. Analysis of the RNA nucleotides was performed and deoxyribonucleotides were present; DNA contamination was estimated to range from about 2 to 8% of the RNA preparations.

Synthesis of hybrid bisnucleoside 5',5"'-P1,P4-tetraphosphates by aminoacyl-tRNA synthetases

Aminoacyl tRNA synthetases, by means of a back reaction, are able to synthesize certain 5', 5"'-P1, P4-bisnucleoside tetraphosphates of biological importance, such as Ap4A. Here it is shown that HisRS and TrpRS (Bacillus stearothermophilus) and AlaRS (E. coli) also synthesize the hybrid compounds Ap4G, Ap4C, and Ap4U. GlnRS (E. coli) is unable to synthesize any of the above compounds. AlaRS synthesizes Ap4U very poorly, and Ap4C and Ap4G almost as effectively as Ap4A. HisRS and TrpRS synthesize Ap4G, Ap4U and Ap3U quite effectively, and Ap4C very poorly. The fact that hybrid bisnucleoside tetraphosphates can be made by the same enzymes, and at rates comparable to Ap4A, suggests that these compounds may also occur in vivo.