In Silico Comparison of the Hemicelluloses Xyloglucan and Glucuronoarabinoxylan in Protecting Cellulose from Degradation

We used a previously developed simulation model of the plant cell wall and its enzymatic degradation to compare the ability of two hemicelluloses, glucuronoarabinoxylan (GAX) and xyloglucan (XG), to protect cellulose microfibrils (CMFs) from attack by cellulose-degrading enzymes. Additionally, we investigated the effect of XG abundance on the degradation rate of CMFs in the presence of the same enzymes. Simulations were run using hypothetical cell wall compositions in which the numbers and arrangement of CMFs and (1,3;1,4)-β-glucan was kept constant, but hemicellulose content was altered. Scenarios considered walls with low and equal numbers of either GAX or XG, and also low, medium and high levels of XG in the absence of GAX. CMF degradation was much lower in walls with GAX than walls with XG, except for early in the simulation when the reverse held, suggesting that XG were protecting CMFs by competitive inhibition. Increasing XG content reduced both the degradation rate of CMFs and the percent of XG degraded, indicating that steric hindrance of enzymes increased with XG density despite XG being degradable. Glucose oligosaccharide breakdown products were analysed on the basis of originating polysaccharide and degree of polymerisation (DP). Hemicellulose type and abundance had significant effects on the amount and profile of breakdown products.