Massive enzyme footballs control sugar metabolism
Neutrons have shown how massive enzyme complexes inside cells have the potential to fine-tune the rate of glucose utilisation and maintain the optimal balance between carbohydrate and fat utilisation by the body. These findings should improve our understanding of diabetes and a range of metabolic and genetic disorders.
A research team at the University of Glasgow led by Prof Gordon Lindsay and Dr Olwyn Byron, in collaboration with neutron scientists at the Institut Laue-Langevin (ILL) in Grenoble, has shown how the pyruvate dehydrogenase complex (PDC) could exert fine control of the rate of sugar metabolism.
PDCs are found within all cell types from bacteria to mammals in which they are known to help regulate the level of sugar in the blood to meet the continuously changing metabolic demands of the body. These massive ‘molecular machines’ composed of multiple copies of 3 distinct enzymes (E1, E2 and E3) have a unique, football-shaped central scaffold, forming a hollow ball with 12 open pentagonal faces. In mammals and other higher organisms, these remarkable structures comprise 60 individual subunits made up of two related proteins: the E2 enzyme, which forms the central framework of the complex, and E3 binding protein (E3BP, originally discovered in Glasgow) which primarily plays a structural role by tethering the E3 enzyme at precise locations on the outside of the central ‘football’, to promote rapid and efficient metabolism. Whilst the overall structure of the central framework was well understood [1], its exact composition was undetermined. Most previous studies had suggested a ratio of 48 metabolic enzyme (E2) subunits to 12 structural (E3BP) subunits.
The Glasgow team, brought together during the PhD of one of the team members – Dr Swetha Vijayakrishnan - synthesised human PDC in bacteria (in itself a considerable technical feat) and identified the location and relative abundance of the subunits through small angle neutron scattering (SANS) studies with colleagues at the ILL in Grenoble, led by Dr Phil Callow. This revealed a new, unexpected ratio of 40:20 in favour of the scaffold (E2) enzyme, whereas experiments on PDC isolated from natural sources (e.g. bovine heart and yeast) confirmed the expected figure of 48:12.
With further mathematical modelling, by Prof Peter Kropholler and Prof David Gilbert (now at Brunel University), the team have shown that their synthesised PDC could vary its composition, with any ratio from 60:0 to 40:20 possible.
Professor Gordon Lindsay, University of Glasgow: “Using neutron scattering at ILL, we have shown the potential of these football structures to vary their composition to allow the most efficient utilisation of sugars by the body and enable precise control of sugar break-down. The next step is to see if this occurs naturally across different tissues of the body and in different living organisms. Our ability to synthesise this massive complex in bacteria also permits us to investigate the consequences of many naturally-occurring mutations in this vital piece of molecular machinery and to address fundamental questions on disease mechanisms that will ultimately be beneficial to patients and clinicians alike through the development of novel therapies and improved treatment regimes.”
Dr Olwyn Byron, University of Glasgow: “This phase of PDC research was supported in large part by the Wellcome Trust 4-year PhD Programme Molecular Functions in Disease at University of Glasgow which excels in recruiting students from very diverse scientific backgrounds and positively fosters multidisciplinary approaches to research. It has also benefitted significantly from grant funding by BBSRC and EPSRC – the latter underpinning the transformational Deuteration Laboratory in Grenoble.”
Professor Andrew Harrison, ILL’s Director for Science: “ILL has a proud history carrying out fundamental research that underpins medical breakthroughs and potential new treatments. The PDC studied by Dr Callow and his colleagues at University of Glasgow is too large for analysis by most other techniques. By using neutrons and the wide range of instruments available at ILL, they have given the medical world a new perspective on diseases that affect thousands of people across the world.”
These findings could provide vital information for future treatments of diseases caused by abnormal blood sugar levels, such as diabetes, and those directly linked to mutations in the PDC that are implicated in a range of genetic, metabolic, neurodegenerative and autoimmune disorders.
Biochemical Journal – accepted, EMBARGOED UNTIL PUBLICATION DATE
Notes for editors
1. The PDC central core framework is made up of two related types of protein subunits, termed E2 and E3BP. The E2 enzyme is the structural and mechanistic heart of the complex and has a unique lipoyl ‘swinging arm’ which has to interact with its two companion enzymes, E1 and E3, during the catalytic cycle. E2 is also responsible for tethering and positioning the E1 enzyme on the central scaffold whereas the second scaffold protein E3BP is primarily a structural protein that is responsible for integrating the E3 enzyme correctly into this large molecular machine.
2. Vijayakrishnan et al (2011) Biochem. J. May 31 (Epub ahead of print) August 1st 2011 is the expected print publication date.
3. About the ILL – the Institut Laue-Langevin (ILL) is an international research centre based in Grenoble, France. It has led the world in neutron scattering science and technology for almost 40 years, since experiments began in 1972. The ILL operates one of the most intense neutron sources in the world, feeding beams of neutrons to a suite of 40 high-performance instruments that are constantly upgraded. Each year 1,200 researchers from over 40 countries visit the ILL to conduct research into condensed matter physics, (green) chemistry, biology, nuclear physics, and materials science. The UK, along with France and Germany is an associate and major funder of the ILL.