Structure of the first natively non-processive pectin methyesterase (from <em>Aspergillus niger</em>): comparison to processive orthologues from plants and bacteria  — ASN Events
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  3. Structure of the first natively non-processive pectin methyesterase (from Aspergillus niger): comparison to processive orthologues from plants and bacteria 

Structure of the first natively non-processive pectin methyesterase (from Aspergillus niger): comparison to processive orthologues from plants and bacteria  (#423)

Geoffrey B Jameson 1 , Lisa M Kent 1 , Trevor S Loo 1 , Gillian E Norris 1 , Laurence D Melton 2 , Davide Mercadante 2 , Martin AK Williams 1
  1. Institute of Fundamental Sciences, Massey University, Palmerston North, Manawatu, New Zealand
  2. School of Chemical Sciences, University of Auckland, Auckland, New Zealand

Pectin methylesterases (PMEs) are deeply involved in plant cell wall modification through hydrolysis of the homogalacturonan O6-methylester groups of pectin. PMEs are also produced by bacteria and fungi to attack plant cell wells. Whereas bacterial and plant PMEs are processive (de-methylesterification proceeds sequentially from one galacturonan residue to the next), fungal PMEs are reportedly non-processive (de-methylesterification occurs randomly).   Through molecular dynamics simulations on variously patterned decasaccharide homogalacturonan (HG) species we have probed the mechanism of processivity1,2  in the structurally well-characterised 3 bacterial PME Erwinia chrysanthemi. Processive PME enzymes can be viewed as uni-directional molecular motors that rectify Brownian motion through diffusion in a biased potential that is periodically lifted by removal of pre-existing substituents on the polymer -- the fuel powering rectification is carried by the substrate itself in its methylester groups and not by an external agency such as ATP or ion gradients.

We report here the biochemical characterisation of the closely homologous PME1 and PME2 from the fungal plant pathogen Aspergillus niger, showing that relative to a plant (orange) PME these enzymes are non-processive.  The plant and bacterial PMEs share ~30% sequence identity with each other and with the fungal PMEs,  The structure of PME2 in completely deglycosylated (PNGaseF-treated) and Endo-H treated forms (N-acetylglucosamine stub) has been determined to better than 1.8 Å resolution.  The beta-helix structure common to a number of carbohydrate-processing enzymes is also observed for PME2.

The plant4,5 and especially the bacterial3 PMEs have substantially longer loops surrounding the substrate-binding site than the fungal PME2, but this is not necessarily the determinant of processivity. Moreover, the fungal PME2 has a strongly negatively charged surface relative to the plant and bacterial orthologues at neutral pH. Parallel molecular dynamics simulations to those performed on the bacterial PME-HG complexes are being conducted on fungal PME2-HG complexes to identify the features controlling processivity/non-processivity in PMEs.   

  1. Mercadante M, Melton LD, Jameson GB, Williams MAK, De Simone A. 2013. Biophysical Journal 104: 1731-1739 (10.1016/j.bpj.2013.02.049).
  2. Mercadante D, Melton LD, Jameson GB, Williams MAK. 2013. Submitted.
  3. Jenkins J, Mayans O, Smith D, Worboys K, Pickersgill RW. 2001. Journal of Molecular Biology 305: 951-960.
  4. Di Matteo A, Giovane A, Raiola A, Camardella L, Bonivento D, De Lorenzo G, Cervone F, Bellincampi D, Tsernoglou D. 2005. Plant Cell 17: 849-858.
  5. Johansson K, El-Ahmad M, Friemann R, Jornvall H, Markovic O, Eklund H. 2002. FEBS Letters 514: 243-249.