Table 3.

Properties of four class IV proteins

	Carboxypeptidase A	Thermolysin	Sonic hedgehog	Glycerol kinase
Ligands	H, H, acidic unibidentate, bound water
	In coils and β-strands. H residues are distant in sequence.	All ligands in α-helices. Classic zinc protease motif: HEX2H	All buried in strands and coils; not a protease but autoproteolytic	Two buried H 15 residues apart in chain F; one H in coil, one H in β-strand; E exposed in coil
His ligand contact	2 Nδ1	2 Nɛ2	1 Nδ1, 1 Nɛ2	2 Nɛ2
His bond lengths	Moderate, 2.08–2.13 Å	Tight, 1.93–1.97 Å	Moderate, 2.06–2.08 Å	Long, 2.44–2.21 Å
2nd shell	In buried coils and β-strands	Mostly in α-helices, partly exposed	Many in β-strands, partly exposed	Partly exposed; mixed in secondary structures
Special residues of 2nd shell	D-142 H-bonds to ligand H-69	D-170 H-bonds to ligand H-142	E-54 H-bonds to ligand H-183	T-95 H-bonds to H-75
Significant clusters	None	{HED} about Zn2+; {ED} about dicalcium	{HED} about Zn2+; {ED} cluster covers the zinc site	None