The three-dimensional structure of recombinant human monoamine oxidase A (hMAO A) as its clorgyline-inhibited adduct is described. Although the chain-fold of hMAO A is similar to that of rat MAO A and human MAO B (hMAO B), hMAO A is unique in that it crystallizes as a monomer and exhibits the solution hydrodynamic behavior of a monomeric form rather than the dimeric form of hMAO B and rat MAO A. hMAO A's active site consists of a single hydrophobic cavity of Ϸ550 Å 3 , which is smaller than that determined from the structure of deprenyl-inhibited hMAO B (Ϸ700 Å 3 ) but larger than that of rat MAO A (Ϸ450 Å 3 ). An important component of the active site structure of hMAO A is the loop conformation of residues 210 -216, which differs from that of hMAO B and rat MAO A. The origin of this structural alteration is suggested to result from long-range interactions in the monomeric form of the enzyme. In addition to serving as a basis for the development of hMAO A specific inhibitors, these data support the proposal that hMAO A involves a change from the dimeric to the monomeric form through a Glu-151 3 Lys mutation that is specific flavin ͉ neurotransmitter ͉ membrane protein ͉ antidepressant target H uman monoamine oxidase A (hMAO A) is an outer mitochondrial membrane-bound flavoenzyme that catalyzes the oxidation of the neurotransmitters serotonin, dopamine, and norepinephrine. Recent studies have demonstrated that a deficiency or low level of expression of this enzyme results in a phenotype of aggressive behavior (1, 2). The elucidation of the 3D structures of human MAO B (hMAO B) (3, 4) (72% sequence identity with hMAO A) and of rat MAO A (rMAO A) (5) (92% sequence identity with hMAO A with no insertions or deletions) has provided insights into the structure and mechanism of these pharmacologically important enzymes. There are several functional properties of hMAO A that differentiate it from rMAO A, despite their high level of sequence identity. hMAO A has been shown to exhibit a 10-fold lower affinity (IC 50 ) than rMAO A for the specific irreversible inhibitor clorgyline (6). Comparisons of the influence of a Phe-208 3 Ile mutation on MAO A from human (7) and rat (8) also show differential effects on activities and sensitivities to irreversible inhibition. Functional differences between hMAO A and rMAO A have been implicated in comparison with their respective sensitivities to phentermine inhibition (9). These differences in properties between hMAO A and rMAO A suggest structural differences exist for these two enzymes.With the development of a high-level expression system for hMAO A in our laboratory (10) and successes with the structural elucidation of hMAO B (3, 4), a collaborative program was established to elucidate the structure of hMAO A by x-ray crystallography. Here, we report the structures of two hMAO A crystal forms and demonstrate structural differences between hMAO A and rMAO A as well as hMAO B. Our data indicate that the considerable literature on MAO A-inhibitor development by using rat models m...

Monoamine oxidase B (MAO-B)is an outer mitochondrial membrane-bound enzyme that catalyzes the oxidative deamination of arylalkylamine neurotransmitters and has been a target for a number of clinically used drug inhibitors. The 1.7-Å structure of the reversible isatin-MAO-B complex has been determined; it forms a basis for the interpretation of the enzyme's structure when bound to either reversible or irreversible inhibitors. 1,4-Diphenyl-2-butene is found to be a reversible MAO-B inhibitor, which occupies both the entrance and substrate cavity space in the enzyme.

The HERG voltage-dependent K+ channel plays a role in cardiac electrical excitability, and when defective, it underlies one form of the long QT syndrome. We have determined the crystal structure of the HERG K+ channel N-terminal domain and studied its role as a modifier of gating using electrophysiological methods. The domain is similar in structure to a bacterial light sensor photoactive yellow protein and provides the first three-dimensional model of a eukaryotic PAS domain. Scanning mutagenesis of the domain surface has allowed the identification of a hydrophobic "hot spot" forming a putative interface with the body of the K+ channel to which it tightly binds. The presence of the domain attached to the channel slows the rate of deactivation. Given the roles of PAS domains in biology, we propose that the HERG N-terminal domain has a regulatory function.