(a) Molecular surfaces of HaNDK, HaBLA, and HaAP. The numbers in parentheses are the ratios of positively charged to negatively charged residues, indicating the acidic residue contents of proteins. (b) Hydrophobic cluster (magenta sticks) formed by the internal hydrophobic residues of HaAP. This cluster may attract a substrate (an aromatic or aliphatic phosphoester) to the catalytic site (white sticks). Mg²⁺ and Zn²⁺ are known to catalyze the enzymatic reaction of alkaline phosphate.