Thesis (MSc)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: Membrane bound cytochrome b5 is a ubiquitous protein with an average molecular weight of 16 kDa. The protein is involved in a number of reactions providing electrons directly to cytochrome P450 enzymes or to other enzymes involved in lipid biosynthesis. It is also known that the protein influences the activities of certain enzymes via an allosteric effect. It has been accepted in the literature that the cytochrome b5 exists primarily in the monomeric form, however, recently it has been shown that it forms homomeric complexes in vivo. In this study, we investigate the cytochrome b5 complex formation using a variety of analytical tools. Cytochrome b5 was isolated from ovine liver microsomes and the purity verified using sodium dodecyl sulphate polyacrylamide gel electrophoresis and electrospray ionisation mass spectrometry. The latter analysis confirmed the presence of a single heme containing protein with Mr=15865 Da, while separation on the polyacrylamide gel revealed oligomeric complex formation with the tetrameric form the most prominent oligomer. Using different and particularly harsh denaturing conditions we found that the observed oligomeric aggregates persisted, indicating highly stable complexes. The most prominent tetrameric aggregate was identified to be cytochrome b5 by mass spectrometric sequencing. Further complex formation studies, using a fluorescent dye (1-anilinonaphthalene-8-sulfonic acid) that interact with hydrophobic cavities formed during oligomerisation, provided evidence of protein assembly in oligomeric complexes or aggregation. The formation of the cytochrome b5 complexes was dependent on ionic strength and protein concentration. Previously it was shown that the hydrophobic membrane anchoring domain plays a pivotal role in the cytochrome b5’s homomeric complexes. Using a peptide (IITTIDSNSS), resembling a portion of this domain, together with circular dichroism we showed more organized structure present for the wildtype peptide vs. a mutated control peptide (LLSSLKAVAV). A modified ELISA interaction assay also revealed that the wild-type peptide had a specific interaction with cytochrome b5, providing further evidence that the membrane anchoring domain plays a role in complex formation. These studies also indicated that a hydrogen bond network in this domain may be important for the formation of the homomeric complexes of cytochrome b5. / AFRIKAANSE OPSOMMING: Membraan-gebonde sitochroom b5 is ’n alomteenwoordige proteïen met ’n gemiddelde molekulêre massa van 16 kDa. Die proteïen is betrokke in reaksies waar dit elektrone direk aan sitochroom P450 ensieme verskaf, sowel as ensieme betrokke in lipiedbiosintese. Dit is ook bekend dat die proteïen die aktiwiteite van sekere ensieme via ’n allosteriese effek beïnvloed. Dit is geredelik in die literatuur aanvaar dat sitochroom b5 as ’n monomeer voorkom, maar daar is kort gelede gerapporteer dat homomeriese komplekse in vivo vorm. In hierdie studie is die sitochroom b5-kompleksvorming ondersoek deur gebruik te maak van verskeie analietiese metodes. Sitochroom b5 is vanuit skaaplewer mikrosome geïsoleer en die suiwerheid met behulp van natrium-dodesiel-sulfaat-poliakrielamied-gel-elektroforese en elektrosproei-ionisasie massa-spektrometrie geverifieer. Met die laasgenoemde bevestig dat ’n enkele heem-bevattende proteïen met Mr =15865 teenwoordig was, terwyl poliakrielamied gel-skeiding kompleksvorming getoon het, met tetrameer as die mees prominente oligomeer. Deur verskeie denaturerings kondisies, intsluitend besondere kondisies, is gevind dat hierdie aggregate behoue bly, wat baie stabiele oligomere aandui. Die mees prominente tetrameriese aggregaat is as sitochroom b5 geïdentifiseer met behulp van massa spektrometriese volgordebepaling. Kompleksvorming is verder bewys deur ’n verdere ondersoek met behulp van ’n fluoresserende kleurstof (1-anilinonaftaleen-8-sulfoonsuur) wat met die hirdofobiese holtes, wat vorm tydens oligomermerisasie, interaksie het. Die kompleksvorming was afhanklik van ioniese sterkte, sowel as proteïenkonsentrasie. Voorheen was dit bewys dat die deurslaggewende faktor in die vorming sitochroom b5 se homomeriese komplekse die hidrofobiese membraan-anker-domein is. Deur gebruik te maak van ’n peptied (IITTIDSNSS) wat lyk soos ’n gedeelte van hierdie domein, tesame met sirkulêre dichroisme, is gewys dat meer georganiseerde struktuur teenwoordig was vir die wilde tipe peptied vs. ’n gemuteerde kontrole peptied (LLSSLKAVAV). ’n Gemodifiseerde ELISAinteraksie- essai het ook getoon dat die wilde-tipe peptied spesifieke interaksie met sitochroom b5 het, ’n verdere bewys dat hierdie membraan-anker-domein ’n rol speel in kompleksvorming. Hierdie studies het ook aangedui dat ’n waterstofbinding netwerk in die domein belangrik kan wees vir die vorming van die homomeriese komplekse van sitochroom b5.

Dissertation (PhD)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: This dissertation describes how the isolation and characterisation of ovine liver cytochrome b5 was accomplished by referring to the following goals achieved in this study: - The optimisation of the isolation and purification procedure for ovine liver microsomal cytochrome b5 in order to obtain sufficient material for aggregation and immunological studies. - The removal of the membrane binding domain of cytochrome b5 by means of tryptic digestion to establish the role of the carboxyl terminal in ovine cytochrome b5 aggregation. - The raising of antibodies against both the trypsin truncated and intact forms of cytochrome b5 to study the aggregation of the protein. - The investigation into the influence of purified cytochrome b5 on steroidogenesis in ovine adrenal microsomes. / AFRIKAANSE OPSOMMING: Die isolering en karakterisering van skaaplewersitochroom b5, soos beskryf in hierdie proefskrif, is uitgevoer deur die volgende doelwitte suksesvol af te handel: - Die optimalisering van die prosedure vir die suksesvolle isolering en suiwering van skaaplewersitochroom b5 ten einde genoegsame hoeveelhede van die suiwer proteïen te hê vir die bestudering van die aggregasie van die proteïen sowel as ‘n immunologiese studie. - Die verwydering van die membraanbindingsdomein van sitochroom b5 om die invloed van die karboksielterminaal op die aggregering van die proteïen te bestudeer. - Die gebruik van sowel die tripties gesnyde as die intakte vorms van sitochroom b5 om ‘n immuunrespons in hase op te wek vir die verkryging van sitochroom b5 spesifieke anti-liggame. - Die gebruik van die gesuiwerde proteïene om die invloed van sitochroom b5 op adrenale steroïdogenese te bestudeer.