It had been shown previously by heterologous expression in Schizosaccharomyces pombe, that the two monosaccharide/H+ symporters HUP1 and HUP2 of Chlorella kessleri differ significantly concerning their substrate specificity: HUP1 transports predominantly D-glucose while HUP2 prefers D-galactose. Several chimeric transporters were constructed and their substrate specificities determined. Surprisingly, it is sufficient to replace the first part of the external loop 1 of the HUP1 symporter by the corresponding portion of HUP2 to improve transport and also to decrease the K(m) value for D-galactose. Additional data indicating the importance of the first loop for substrate recognition and binding are discussed.
Will, A., & Tanner, W. (1996). Importance of the first external loop for substrate recognition as revealed by chimeric Chlorella monosaccharide/H+ symporters. FEBS Letters, 381(1–2), 127–130. https://doi.org/10.1016/0014-5793(96)00097-X