**2. Structure of avian muscarinic receptors using homology modeling**

So far, there are no reported structures for avian muscarinic receptors in protein data bank or any other public domain sites. We used homology modeling to build the receptor structure using protein-BLAST search (Sequence ID for this search are provided with the individual structures - see **Figure 1**). Homology modeling is used to determine three-dimensional structure of proteins using its amino acid sequence. It is a model used for computational structure prediction of proteins. Afterwards the SWISS-MODEL service (https://swissmodel.expasy.org/) was used to model the tertiary structure using suggested template protein structure. Discovery Studio Client was

*Avian Muscarinic Receptors: An Update DOI: http://dx.doi.org/10.5772/intechopen.111720*

#### **Figure 1.**

*Images of muscarinic acetylcholine receptor subtypes reported in chicken (Gallus gallus domesticus) homology modeled using Swiss model (https://swissmodel.expasy.org/).*

used to visualize the final model (**Figure 1**). In the picture the visual similarities and differences between M2, M3, M4, and M5 are visible. By doing nucleotide sequence alignment, it has been shown that there are not many similarities between each of the muscarinic receptors, though acetylcholine can bind with all the receptors. This may be due to the similar sequences in the ligand binding domains of all the four receptors.
