The invention relates to a glucose binding protein comprising amino acid mutations relative to the wild type sequence at the following positions: (i) H 152, (ii) A213; and (iii) L238 wherein the mutation at position H 152 is H152C. The invention further relates to such a glucose binding protein comprising the mutations H152C, A213R and L238S, in particular when linked to an environmentally sensitive dye such as badan.
