Protein-Polysaccharide Interaction

The ability to visualise protein-polysaccharide interactions has led to the discovery of novel protein-polysaccharide complexes formed by cell-wall polysaccharides, new insights into the mechanisms of action of enzymes and novel ways to probe heterogeneity of polysaccharide structures.

Both sugar beet pectin and water-soluble arabinoxylans are unusual in their ability to act as emulsifiers. In both cases this has been attributed to residual protein that is difficult to remove from the polysaccharide. In both cases AFM studies have revealed protein attached to the polysaccharide.

For sugar beet pectin complexes the protein is attached at one end of the carbohydrate chain. The pectin chain can be wound around the protein or extended, and in the helical form, when in contact with the substrate.

Water-soluble wheat pentosans contain protein (P)-arabinoxylan (AX) complexes.

In both cases the role played by these complexes in cell wall structure or its assembly is still unknown.

Most cell wall polysaccharides are heterogeneous structures. The fact that proteins can be visualised bound to the chains provides a means of mapping such structures. Site-directed mutagenesis can be used to knockout catalytic activity whilst retaining the binding specificity of enzymes. This has been demonstrated through detecting the binding of inactivated xylanases to arabinoxylans. The binding patterns can be analysed mathematically to distinguish between random and non-random distributions of binding sites along the molecules.

In principle the methods could be used to analyse the mode of action of enzymes. In some cases the complexes formed between enzymes and polysaccharides provide clues to the mode of action.

A model of the ring-like complexes formed between amylose, and the starch-binding domain (SBD) of glucoamylase, has suggested a novel molecular mechanism for the role of the SBD in enabling glucoamylase to degrade crystalling starch.

Further Reading:

Kirby AR, Alistair J. MacDougall AJ & Morris VJ
Sugar Beet Pectin – Protein Complexes. Food Biophysics 1 (1) 51-56 (2006).

Morris VJ, Gunning AP, Faulds CB, Williamson G & Svensson B.
AFM images of complexes between amylose and Aspergillus niger glucoamylase mutants, native and mutant starch binding domains: a model for the action of glucoamylase. Starke 57 (2005) 1-7.

Adams EL, Kroon PA, Williamson G, Gilbert HJ & Morris VJ
Inactivated enzymes as probes of the structure of arabinoxylans as observed by atomic force microscopy. Carbohydr. Res. 339 (2004) 579-590.

Adams LL, Kroon P, Williamson G & Morris VJ
Characterisation of heterogeneous arabinoxylans by direct imaging of individual molecules by atomic force microscopy. Carbohydrate Research 338 (2003) 771-780.

Gunning AP, Giardina TP, Faulds CB, Juge N, Ring SG, Williamson G & Morris VJ.
Surfactant mediated solubilisation of amylose and visualisation by atomic force microscopy. Carbohydrate Polymers 51 (2003) 177-182.

Juge N, Le Gal-Coëffet M-F, Furniss CSM, Gunning AP, Giardina T, Kramhøft, B, Morris VJ, Svensson B & Williamson G.
The starch binding domain of glucoamylase from Aspergillus niger: overview of its structure, function, and role in raw starch hydrolysis. Biologia 57 (2002) 230-245.