Homology model of feline pancreatic GK protein. The protein is shown bound to molecules of ATP and glucose with the binding site and catalytic pocket facing outward. ATP (not shown) binds deeper in the pocket than glucose (shown in green). The residues involved in ATP binding (dark blue) are completely conserved in the feline GK protein, as are the residues involved in glucose binding (orange). Some residues are involved in both ATP and glucose binding (pink). Inset – The face of the GK molecule opposite the binding pocket is shown. The N-terminal portion of the GK molecule (light blue) contains three of the five non-conserved amino acid substitutions found in feline pancreatic GK. Feline GK has trp-arg (shown in purple and green, respectively) at positions 35 and 36 while other species have arg-arg or ser-arg in these same positions. As a result of the presence of trp at position 35 and ala at position 28 (another non-conserved feline variant) feline GK lacks a salt bridge that is present in human GK.