User:Martha Blakely/Sandbox 1

From Proteopedia

Revision as of 05:58, 25 June 2010

(MAO-B) is a mitochondrial outer-membrane flavoenzyme. It is one of two isozymes (MOA-A is the other), which catalyzes the oxidative deamination of amine neurotransmitters, including serotonin, dopamine, and epinephrine. The flavin-dependent enzymes use O₂ as an electron acceptor in the catalytic pathway which includes hydrogen peroxide, H₂O₂, as a product. Increased H₂O₂ levels promote apoptotic signaling of cells. Thus, researchers have found that increased levels of MAO-B, whose levels in the brain increase at least 3-fold on aging, are related to decreased levels of neuronal cells. The cells in the brain particularly targeted are dopamine-producing cells, which results in the development of Parkinson’s Disease. A popular example related to the development of Parkinson symptoms based on MAO-B levels is the oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to 1-methyl-4-phenylpyridinium (MPP+) which destroys glial cells. The inhibition of MAO-B prevents against this cell death behavior.

Inhibitors of MAO-B such as deprenyl and rasagiline have been developed in order to protect the brain from neurological disorders. Deprenyl is used in L-dopa therapy which acts as treatment and protection for Parkinson's and pre-Parkinson's patients.

Structure

Structurally, MAO-B is a dimer with each monomer composed of 520 amino acids. The sequence forms a helix on each monomer that is responsible for the attachment to the outer-membrane of mitochondria. This α-helix resides in the lipid bilayer. In addition to the C-terminal helices, other hydrophobic side chains such as also contribute to attachment.

The active site of MAO-B is a flat hydrophobic cavity that is separated into two parts, the entrance cavity and the substrate cavity. The dual nature of the active site allows for binding of differently sized substrates. Some are large enough to fill both cavities, some only fill the substrate cavity, and others are too small to fit either. The wide range of substrate size can partially be contributed to a boundary between the two cavities. Four residues () form this boundary between cavities. Ile199 is a gating residue that can be in the “open” or “closed” positions.

Entrance into the connected cavities is determined by the movement of a loop of amino acids () located at the protein surface near the membrane attachment. This loop prevents solvent from entering the active site. The loop’s location is near the region of the protein that binds with the mitochondrial outer-membrane. This proximity implies that passage into the active site can only occur near the membrane.

FAD cofactor with an empty N5 bond.

Each monomer has its own covalently bonded.

Difference from MAO-A

Both are flavoenzymes found attached to the outer-membrane of mitochondria. The amino acid sequences of the two are about 70% similar and are encoded by separate genes on the X-chromosome. Both catalyze the oxidative deamination of amine neurotransmitters, but the enzyme selectivity of the neurotransmitters varies. MAO-A is a monomer, whereas MAO-B is a dimer. Depression and mood swings are related to higher levels of MAO-A, rather than diseases like Parkinson’s and Alzheimer’s that are associated with higher levels of MAO-B. Serotonin, a neurotransmitter related to mood elevation, is specific to MAO-A. The residues that correspond to binding are also different between these two enzymes.

Catalytic Pathway

The oxidative deamination of amine neurotransmitters is catalyzed by both monoamine oxidases. The image above shows the two generally proposed and accepted catalytic pathways for this oxidation; the lower loop is the case for most substrates. In this loop, the enzyme-imine compound (E.FAD_red--Imine) reacts with oxygen (O₂) to create H₂O₂ and the oxidized enzyme-imine compound (E.FAD_ox). The imine is then reduced to the ammonium ion (NH₄⁺) and its respective aldehyde. The cycle goes around the loop another time when an amine (S) binds to the active site within the enzyme (E.FAD_ox + S).

Hydrogen peroxide is shown as a product in this catalytic pathway. Due to hydrogen peroxide's association with health problems such as Parkinson's disease and Alzheimer's disease, MAO inhibitors have recently been created with possible neuroprotective functions.

Inhibition

References, for further information on Monoamine oxidase B

• Binda, Newton-Vinson, Hubalek, Edmondson, and Mattevi (2002) "Structure of human monoamine oxidase B, a drug target for the treatment of neurological disorders", Nature Structural Biology 9: 22-26.
• Edmondson, Binda, Wang, Upadhyay, Mattevi (2009) "Molecular and Mechanistic Poperties of the Membrane-Bound Mitochondrial Monoamine Oxidases", Biochemistry 48: 4220-4230.
• Youdim, Edmondson, Tipton (2006) "The therapeutic potential of monoamine oxidase inhibitors", Nature Reviews 7:295-309.