Functional and structural role of amino acid residues in the odd-numbered transmembrane alpha-helices of the bovine mitochondrial oxoglutarate carrier.

TitleFunctional and structural role of amino acid residues in the odd-numbered transmembrane alpha-helices of the bovine mitochondrial oxoglutarate carrier.
Publication TypeJournal Article
Year of Publication2007
AuthorsCappello, AR, Miniero, DV, Curcio, R, Ludovico, A, Daddabbo, L, Stipani, I, Robinson, AJ, Kunji, ERS, Palmieri, F
JournalJ Mol Biol
Volume369
Issue2
Pagination400-12
Date Published2007 Jun 01
ISSN0022-2836
KeywordsAnimals, Biological Transport, Cattle, Cysteine, Ketoglutaric Acids, Membrane Transport Proteins, Mitochondria, Mitochondrial Proteins, Models, Molecular, Mutagenesis, Site-Directed, Protein Structure, Secondary, Protein Structure, Tertiary, Sulfhydryl Reagents
Abstract

The mitochondrial oxoglutarate carrier (OGC) plays an important role in the malate-aspartate shuttle, the oxoglutarate-isocitrate shuttle and gluconeogenesis. To establish amino acid residues that are important for function, each residue in the transmembrane alpha-helices H1, H3 and H5 was replaced systematically by a cysteine in a fully functional mutant carrier that was devoid of cysteine residues. The transport activity of the mutant carriers was measured in the presence and absence of sulfhydryl reagents. The observed effects were rationalized by using a comparative structural model of the OGC. Most of the residues that are critical for function are found at the bottom of the cavity and they belong to the signature motifs P-X-[DE]-X-X-[KR] that form a network of three inter-helical salt bridges that close the carrier at the matrix side. The OGC deviates from most other carriers, because it has a conserved leucine (L144) rather than a positively charged residue in the signature motif of the second repeat and thus the salt bridge network is lacking one salt bridge. Incomplete salt-bridge networks due to hydrophobic, aromatic or polar substitutions are observed in other dicarboxylate, phosphate and adenine nucleotide transporters. The interaction between the carrier and the substrate has to provide the activation energy to trigger the re-arrangement of the salt-bridge network and other structural changes required for substrate translocation. For substrates such as malate, which has only two carboxylic and one hydroxyl group, a reduction in the number of salt bridges in the network may be required to lower the energy barrier for translocation. Another group of key residues, consisting of T36, A134, and T233, is close to the putative substrate binding site and substitutions or modifications of these residues may interfere with substrate binding and ion coupling. Residues G32, A35, Q40, G130, G133, A134, G230, and S237 are potentially engaged in inter-helical interactions and they may be involved in the movements of the alpha-helices during translocation.

DOI10.1016/j.jmb.2007.03.048
Alternate JournalJ. Mol. Biol.
Citation Key10.1016/j.jmb.2007.03.048
PubMed ID17442340
Grant ListMC_U105663139 / / Medical Research Council / United Kingdom
MC_U105674181 / / Medical Research Council / United Kingdom