TABLE 1.

The nine current subtypes in the AHS are distinguished by their conserved metal binding ligands

Subtype	Metal(s)	Metal position(s)	Representative PDB code(s)	Residue(s)a in strand:
				1	2	3	4	5	6	7	8
I	Zn, Ni	α, β	1HZY, 3MTW	HxH			K	H	H		D
II	Zn	α, β	1BF6	HxH			E	H	H		D
III	Zn, Fe	α	1A4M, 4DZH	HxH				H	hc		D
IV	Fe	β	1O12	hxhb		E		H	H		dc
V	Zn	β	1M7J	hxhb	C			H	H		dc
VI	Zn	α, β	1ITQ	HxD		E		H	H		dc
VII			1J5S	HxH		H					dc
VIII	Znd	β	4WGX, 4WHB	NxH			kc	H	hc		D
IX	Zn	α	3LS9	HxH				H	hc		T

^aUppercase amino acid code letters indicate ligands that fulfill their characteristic roles; lowercase letters indicate amino acids that differ from these roles. Table modified from Khurana et al. (6).

^bThese residues are present in the active site but do not appear to ligate a metal. In some cases, they can accommodate a second metal ion, but it is not required for catalysis.

^cHydrogen bonds link these residues to the hydrolytic water molecule.

^dDespite the previous classification of subtype VIII member MolA as a Co metalloenzyme (7), we have concluded that MolA is more likely to be a Zn metalloenzyme.