Abstract:
This thesis explores the role of glycosylated (Gal-Hyl) hydroxylysine residues for the structural stability of the collagenous domain of adiponectin (Adpn), a potential therapeutic for Type II diabetes. These Gal-Hyl residues have been shown to play a key role in the formation of bioactive high molecular weight oligomers (HMW) of Adpn. Notably, Type II diabetics have been shown to have lower levels of HMW of Adpn compared to healthy individuals. However, due to the problems associated with recombinant synthesis of mammalian Adpn, an in-depth study of the structural importance of the Gal-Hyl residues has not been possible. Furthermore, although the recent total chemical synthesis of Adpn incorporating Gal-Hyl residues has been reported, the published synthesis was long, complex and ultimately low-yielding. This thesis utilises neoglycoside mimetics of Gal-Hyl to generate neoglycopeptide mimetics of the collagenous domain of Adpn through the use of Fmoc- solid phase peptide synthesis. The structural properties and thermal stabilities of the resultant neoglycopeptides were then studied using circular dichroism techniques. This thesis describes the syntheses of three neoglycopeptide families, for which the serine analogues of the lysine neoglycopeptides were also prepared in order to investigate the effect of the glycan-peptide backbone through-space distance on the secondary structure of the neoglycopeptide. Chapter Three outlines the synthesis of short 18-mers corresponding to residues 66-83 of the collagenous domain of Adpn. Chapter Four describes the synthesis of collagen model peptides (CMPs) which were employed to promote triple helical formation of the resultant neoglycopeptides. Chapter Five describes the synthesis of hybrid neoglycopeptides incorporating both the short 18-mer Adpn sequence as well as the CMP sequence. The main conclusions of this thesis are that both unglycosylated and glycosylated lysine residues play a fundamental structural role in the collagenous domain of Adpn. Furthermore, the degree of glycosylation of the lysine residues is integral to the structural stability of Adpn. There may be an ‘optimal level’ of glycosylation for collagenous peptides, above which the secondary structure of the peptide is significantly altered. These conclusions could have major implications on the understanding of the forces that underpin the structure and function of many collagenous peptides.