Biological systems are dependent upon a complex array of interactions between molecules. Our laboratory’s primary focus is on understanding the factors that govern specificity in these interactions. Our group investigates molecular structure/function relationships and interactions relevant to major diseases such as diabetes, obesity and cancer. We have a well-established program of research into several important protein systems:
1. The biotin-dependent enzyme Pyruvate Carboxylase (PC) catalyses the ATP-dependent carboxylation of pyruvate to yield oxaloacetate that is essential for the continuing function of the Krebs tricarboxylic acid cycle in both its oxidative and synthetic modes. Thus PC serves vitally important roles in liver and kidney for gluconeogenesis, in liver, adipose tissue and lactating mammary gland for lipogenesis, in brain and central nervous tissue for glucose oxidation and neurotransmitter syntheses, in pancreatic islets for insulin secretion and in brown adipose tissue for thermogenesis. Children born with genetic defects in both copies of the gene encoding PC face the fatal consequences or the severely debilitating symptoms of hypoglycaemia, lactic acidaemia, hyperammonaemia, citrullinaemia, hyperlysinaemia, delayed development and psychomotor retardation.
A project on the structure & function of pyruvate carboxylase is conducted in collaboration with Professor W.W. Cleland [University of Wisconsin-Madison, USA], Assoc.-Prof. Paul Attwood [Univ. Western Australia], Dr Martin St.Maurice [Marquette Univ., Milwaukee, WI, USA] and Dr Sarawut Jitrapakdee [Mahidol Univ. Bangkok, Thailand], and is supported by grants from the National Institutes of Health, USA, and the Australian Research Council. A project on pyruvate carboxylase's role in insulin secretion is conducted in collaboratione with Professor Michael MacDonald [University of Wisconsin-Madison, USA] and Dr Sarawut Jitrapakdee [Mahidol Univ. Bangkok, Thailand].
2. Biotin Protein Ligase, the enzyme that catalyses, with exquisite specificity, the covalent attachment of the biotin prosthetic group [Vitamin H] to the apoenzyme forms of the key metabolic enzymes dependent on this moiety, such as Pyruvate Carboxylase. The biotin prosthetic group is absolutely required for the activity of members from this enzyme family as it directly participates in the binding and transfer of carbon dioxide between metabolites.
Although BPLs are ubiquitously present in all living species and catalyse a reaction that has been conserved throughout evolution, there are significant differences in the BPLs between species. For example, the most intensely characterised BPL is the BirA protein from the bacteria Escherichia coli.
The enzymes from mammalian and fungal species have also been investigated and, interestingly, are at least twice the size of BirA. The significance of this additional amount of protein structure is not known and is the focus of investigation in our group. This is being investigated using a variety of molecular biology, protein chemistry and structural biology techniques in order to understand the relationship between the structure and functions of BPL.
This project is conducted in collaboration with Assoc.-Prof. Grant Booker [MBS], Assoc-Prof. Renato Morona [MBS], Dr Steven Polyak [MBS], Professor Andrew Abell [Chemistry], Assoc.-Prof Matthew Wilce [Biochemistry, Monash University] and Professor John Turnidge [WCH, Adelaide] and is supported by grants from NHMRC, University of Adelaide, & BioInnovationSA.
3. The Insulin-like Growth Factors (IGFs), their Receptors and Binding Proteins IGF-I and IGF-II are small (~7.5kDa) proteins that share a high degree of sequence and structural similarity with insulin. Both IGFs are potent mitogenic and anti-apoptotic factors that play vital roles in normal growth and development. These actions are mediated by IGF binding to and activating the type-1 IGF receptor (IGF-1R) on the cell surface triggering its intracellular tyrosine kinase domain to initiate a cascade of pleiotropic signals.
IGF-I is regulated by and mediates most of the growth-promoting effects of growth hormone, whereas IGF‑II is largely growth hormone-independent. IGFs have endocrine, paracrine and/or autocrine actions in most tissues. Dysregulation of the IGF system has been implicated in a wide range of disease processes including various cancers, atherosclerosis, neuromuscular disease and diabetic complications, acromegaly (due to IGF-I overproduction) and leprechaunism (an IGF-I deficiency).
The type 1 insulin like growth factor receptor (IGF-1R) is a tyrosine kinase receptor involved in the control of normal cell growth, differentiation, proliferation and migration. The type-2 IGF receptor is a dual function protein that binds IGF-II as well as proteins labelled with mannose-6-phosphate. Ligand binding results in internalisation and trafficking to the lysosome, thereby providing a mechanism to reduce IGF-II levels in the body. Loss of heterozygosity of the IGF2R leading to decreased expression of the receptor has been associated with a range of cancers including ovarian and liver.
A family of six structurally related, high-affinity IGF binding proteins (IGFBPs) modulates the actions of IGFs (1-3). IGFBPs bind almost all of the circulating IGFs, thereby greatly prolonging their circulating half-lives and regulating their passage into tissues. The IGFBPs differ in their relative binding affinities for IGF-I and IGF-II, sites of expression and regulation, and therefore constitute an extracellular mechanism for ‘fine tuning’ IGF actions.
Projects in this field are conducted in collaboration with Dr Briony Forbes [MBS], Assoc.-Prof. Grant Booker [MBS], Dr Peter Hoffmann [Adelaide Proteomics Centre], Professor Shaun McColl [MBS], Professor Ray Norton [WEHI, Melbourne], Professor Leon Bach [Monash Univ.], Prof. Zee Upton & Dr Gary Shooter [QUT, Brisbane], Dr C. Owczarek & Dr E. Maraskovsky [CSL, Melbourne], Professor Jonathan Whittaker [Case-WRU, Cleveland, OH, USA] and Professor Pierre De Meyts [Hagedorn Inst., Copenhagen, Denmark].
