Human Monoamine Oxidase B
Created by Kayla Chen
Human Monoamine Oxidase B (MAO-B, PDB ID=1gos) is an outer mitochondrial membrane-bound flavin-containing enzyme that is a well-known target for antidepressant and neuroprotective drugs. Specifically, MAO-B is an enzyme that catalyzes the oxidative deamination of arylalkylamine neurotransmitters and it is used as a target for a number of clinically used drug inhibitors (1). The enzyme binds to outer-membranes through a
C-terminal transmembrane helix and apolar loops located at various positions in the sequence of the enzyme. MAO-B contains a prosthetic group adenine dinucleotide (FAD) that is covalently bound to residue C397 by an
8 alpha-(S-cysteinyl)-riboflavin linkage(1). MAO-B's associated ligands are FAD and N-[(E)-Methyl](Phenyl)-N-[(E)-2-Propenylidene]Methanaminium (NYP), and it contains two subunit structures, namely
subunit A and subunit B. NYP's role in MAO-B is unknown, but
Flavin adenine dinucleotide (FAD) is a redox cofactor that is involved in important reactions of metabolism; when FAD is reduced to FADH2, it accepts two hydrogen atoms and undergoes a net gain of two electrons. Subunit A and subunit B essentially contain the same amino acid sequences, and each of the subunit contains a
covalently bound flavin (
space-filling model).
MAO-B plays a decisive role in some psychiatric and neurological disorders, including depression and Parkinson's diseases. Because inhibition of MAO-B increases the level of neurotransmitters in the central nervous system, searching for the effective inhibitors represents one important approach to developing novel drugs to treat such illnesses (1). MAO-B shows its aromatic amino acid residues orient approximately perpendicular to the flavin ring, suggesting a functional role in catalysis. Amino residues Tyr 398 and Tyr 435 form an
aromatic cage to provide the recognition site for the substrate amino group on MAO-B (2). This structure provides a framework to investigate the catalytic mechanism of the protein, and by understanding the differences between MAO-A and MAO-B isoforms (i.e. two proteins have the same function but are encoded by different genes, resulting in small differences of their sequences), specific inhibitors of
MAO-A (PDB ID=2z5y) and MAO-B can be developed for clinical use as antidepressants and neuroprotective drugs. A
superimposed image of MAO-A and MAO-B is shown.
According to the Dali Server search, MAO-B and MAO-A share significant similarities in their tertiary structures (3). Monoamine oxidase A is also located in the outer-membrane of the mitochondria of neurons as MAO-B, and MAO-A initiates the degradation of neurotransmitters of dopamine, epinephrine and norepinephrine by converting amine groups into aldehyde groups (4). Studies have shown that human's MAO-A activity level is altered when people acquire mood disorders, and lower activity level of the protein has been linked to the cause of aggressive behaviors such as committing crime (4).
The functional role of the C-terminal transmembrane helix in MAO-B has been of biological interest. The protein region responsible for
membrane attachment ( is formed by the C-terminal amino acids 461-520 (5). The
hydrophobic environments (represented in blue) of residues from Glu-461 to Thr-500 raise the pKa values of their side-chain carboxyl groups, making it possible for these carboxyls to accept protons as they are transported across the membrane. Specifically, studies showed that the C-terminal 29 amino acid residues in MAO-B are responsible for targeting and anchoring the protein to the mitochondrial outer-membrane. A C-terminal truncation leads to a significant decrease in MAO-B catalytic activity, but it does not produce any significant changes in inhibitor specificity. Therefore, the C-terminal anchoring for MAO-B must be an important component to study for MAO-B's biological functions. The C-terminal transmembrane helix departs perpendicularly from the base of the structure in a different way with respect to other monotopic membrane proteins. Several apolar loops exposed on the protein surface are located in proximity of the C-terminal helix, providing additional membrane-binding interactions. One of these loops (residues 99-112) in both subunits A and B also functions in opening and closing the
active site cavity on MAO-B, suggesting that the membrane may have a role in controlling substrate binding (6).
The binding of substrates or inhibitors to MAO-B involves an initial negotiation of a protein loop occurring near the surface of the membrane and two hydrophobic cavities: an "entrance" cavity and an "active site" cavity. 1, 4-Diphenyl-2-butene is found to be a reversible MAO-B inhibitor, which occupies both cavities in the enzyme. Comparison of these two structures identifies Ile-199 as a "gate" between the two cavities (7). The
peptide bond (
space-filling model) between the flavin-substituted Cys-397 and Tyr-398 is in a cis conformation, which creates the proper orientation of the phenolic ring of Tyr-398 in the active site. The flavin ring exists in a twisted non-planar conformation, which is observed in the oxidized form as well as in both the N(5) and the C(4a) adducts (i.e., the chemical product of an addition reaction between two compounds). The active site cavities of MAO-B are highly apolar, but hydrophilic areas exist near the flavin ring and direct the amine moiety of the substrate for binding and catalysis. By comparing different inhibitor-enzyme complexes, small conformational changes can be observed (7).
The crystal structures of MAO-B are in complex with four of the N-propargylaminoindan class of MAO covalent inhibitors (rasagiline, N-propargyl-1(S)-aminoindan, 6-hydroxy-N-propargyl-1(R)-aminoindan, and N-methyl-N-propargyl-1(R)-aminoindan). With the extended propargyl chains covalently bound to the flavin and the indan rings located in the rear of the substrate cavities, Rasagiline, 6-hydroxy-N-propargyl-1(R)-aminoindan, and N-methyl-N-propargyl-1(R)-aminoindan have essentially the same conformation (8). Four ordered water molecules make up an integral part of the active site on MAO-B and establish H-bond interactions to inhibitor atoms. These structural studies provide a guide for future drug designs to improve selectivity and efficacy by introducing appropriate substituents on rasagiline - MAO-B complexes (8).