**1. Introduction**

We had been acquainted with the term beta-coronavirus for about two decades when we first encountered the Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) outbreak that emerged in 2002, infecting about 8000 people with a 10% mortality rate [1]. It was followed by the emergence of the Middle East Respiratory Syndrome Coronavirus (MERS-CoV) in 2012 with 2300 cases and mortality rate of 35% [2]. The third outbreak, caused by SARS-CoV-2, was first reported in December 2019 in China, Wuhan province, which rapidly took the form of a pandemic [3–5]. To date, this new human coronavirus has affected 65.5 million people worldwide and is held accountable for over 1.5 million deaths [6]. SARS-CoV-2 is an enveloped single-stranded plus sense RNA virus whose genome is about 30 kb in length, and which encodes for 16 non-structural proteins, four structural, and six accessory proteins [7]. The four major structural proteins which play a vital role in viral pathogenesis are Spike protein (S), Nucleocapsid protein (N), Membrane protein (M), and Envelope protein (E) [8,9]. SARS-CoV-2 infection is mainly characterized by pneumonia [10]; however, multi-organ failure involving myocardial infarction, hepatic, and renal damage is also reported in patients infected with this virus [11]. SARS-CoV-2 binds to the Angiotensin-converting enzyme 2 (ACE2) receptor on the host cell surface via its S protein [12,13]. ACE2 plays an essential role in viral attachment and entry [14,15]. The study of the interaction of ACE2 and S protein is of utmost importance [16–18].

The S1 subunit of the S protein has two domains, the C-terminal and the N-terminal domains, which fold independently, and either of the domains can act as Receptor Binding Domain (RBD) for the interaction and binding to the ACE2 receptor widely expressed on the surface of many cell types of the host [19,20]. The human ACE2 protein is 805 amino acids long, containing two functional domains: the extracellular N-terminal claw-like peptidase M2 domain and the C-terminal transmembrane collectrin domain with a cytosolic tail [21]. The RBD of the S protein binds to three different regions of ACE2, which are located at amino acids (aa) 24–42, 79–84, and 330–393 positions present in the claw-like peptidase domain of ACE2 [14]. These binding regions are designated in our study as domains D1, D2, and D3, respectively. ACE2 modulates angiotensin activities, which promote aldosterone release and increase blood pressure and inflammation, thus causing damage to blood vessel linings and various types of tissue injury [22]. ACE2 converts Angiotensin II to other molecules

and reduces this effect [23]. However, when SARS-CoV-2 binds to ACE2, the function of ACE2 is inhibited and, in turn, leads to endocytosis of the virus particle into the host cell [24].

Zoonotic transmission of this virus from bat to human and random mutations acquired by SARS-CoV-2 during human to human transmission has also empowered this virus with the ability to undergo interspecies transmission, and, recently, many cases have been reported stating that different species can be infected by this virus [25,26].

In this study, we aim to determine the susceptibility of other species, whether they bear the capability of being a possible host of SARS-CoV-2. We chose nineteen different species (*Bos taurus, Capra hircus, Danio rerio, Equus caballus, Felis catus, Gallus gallus, Homo sapiens, Macaca mulatta, Manis javanica, Mesocricetus auratus, Mustela putorius furo, Pelodiscus sinensis, Pteropus alecto, Pteropus vampyrus, Pan troglodytes, Rattus norvegicus, Rhinolophus ferrumequinum, Salmo salar,* and *Sus scrofa*) and analyzed the ACE2 protein sequence from eighteen non-human species in relation to the human ACE2 sequence and determined the degree of variability by which the sequences differed from each other. We performed a comprehensive bioinformatics analysis in addition to the phylogenetic analysis based on full-length sequence homology, polarity along with individual domain sequence homology and secondary structure prediction of these protein sequences. These findings could have emerged to six distinct clusters of nineteen species based on the collective analysis and thereby provided a prediction of the interspecies SARS-CoV-2 transmission.
