User:Eran Hodis/Sandbox8
General Structure
There are two distinct classes of HMGRs, class I, which is only found in eukaryotes and are membrane bound and class II, which is found in prokaryotes and are soluble. [1] HMGR contains 8 transmembrane domains, which have yet to be successfully crystallized, that anchor the protein to the membrane of the endoplasmic reticulum. [2] The catalytic portion of human HMGR forms a tetramer, with the individual monomers winding around each other. [2] Within the tetramer, the monomers are arranged into two dimers, each of which contains two active sites which are formed by residues form both monomers. Each monomer contains three domains , the N-domain, the L-Domain, and the S-Domain. The L-domain is unique to HMGRs while the S-domain, which forms the binding site for NADP, resembles that of ferredoxin. The S and L domains are connected by a “cis-loop” which is essential for the HMG-binding site. [2] Salt bridges between residues R641 and E782 as well as hydrogen bonds between E700 and E700 on neighboring monomers compliment the largely hydrophobic dimer-dimer interface. [2]
Substrate Binding & Catalytic Mechanism
The HMG-CoA and NADPH molecules make numerous contacts with the L and S domains in forming the four active sites. The CoA is located in a positively charged pocket near the enzyme surface, with the pantothenic acid moiety extending into the interior of the protein. Tyrosine 479 forms a hydrophobic lid over the CoA adenine base, shielding the extended binding pocket from solution. The NADPH binding site is formed primarily by the S-domain with a loop region playing a critical role in binding. [2]
The HMG binding pocket is the site of catalysis in HMGR. The“cis-loop” that bends over the top of HMG is a critical structural element of this binding site. Residues E559 and D767 and are positioned in the active site as is K691 which is only 2.7 angstroms from the HMG O2 carbonyl oxygen. It is this K691 that presumably stabilizes the negatively charged oxygen on the first mevaldyl-CoA intermediate. [2] The mevaldyl CoA intermediate is subsequently converted to Mavaldehyde with added stabilization from H866, which is within hydrogen bonding distance of the thiol group. It is then believed that the close proximity of E559 and D767 increases the pKA of E559, allowing it to be a proton donor for the reduction of mevaldehyde into mevalonate. [2]
- ↑ Istvan ES, Deisenhofer J. Structural mechanism for statin inhibition of HMG-CoA reductase. Science. 2001 May 11;292(5519):1160-4. PMID:11349148 doi:10.1126/science.1059344
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Roitelman J, Olender EH, Bar-Nun S, Dunn WA Jr, Simoni RD. Immunological evidence for eight spans in the membrane domain of 3-hydroxy-3-methylglutaryl coenzyme A reductase: implications for enzyme degradation in the endoplasmic reticulum. J Cell Biol. 1992 Jun;117(5):959-73. PMID:1374417
