COVID-19 infection: Origin, transmission, and characteristics of human coronaviruses

Shereen, Muhammad Adnan; Khan, Suliman; Kazmi, Abeer; Bashir, Nadia; Siddique, Rabeea

The genome of the SARS-CoV-2 has been reported over 80% identical to the previous human coronavirus (SARS-like bat CoV) [34] . The Structural proteins are encoded by the four structural genes, including spike (S), envelope (E), membrane (M) and nucleocapsid (N) genes. The orf1ab is the largest gene in SARS-CoV-2 which encodes the pp1ab protein and 15 nsps. The orf1a gene encodes for pp1a protein which also contains 10 nsps [34] [35] [36] . According to the evolutionary tree, SARS-CoV-2 lies close to the group of SARS-coronaviruses [37, 38] (Fig. 5 ). Recent studies have indicated notable variations in SARS-CoV and SARS-CoV-2 such as the absence of 8a protein and fluctuation in the number of amino acids in 8b and 3c protein in SARS-CoV-2 [34] (Fig. 4) . It is also reported that Spike glycoprotein of the Wuhan coronavirus is modified via homologous recombination. The spike glycoprotein of SARS-CoV-2 is the mixture of bat SARS-CoV and a not known Beta-CoV [38] . In a fluorescent study, it was confirmed that the SARS-CoV-2 also uses the same ACE2 (angiotensin-converting enzyme 2) cell receptor and mechanism for the entry to host cell which is previously used by the SARS-CoV [39, 40] . The single N501T mutation in SARS-CoV-2's Spike protein may have significantly enhanced its binding affinity for ACE2 [33] .