The upgrade of D-xylose, the most abundant pentose, to value-added biochemicals is economically important to next-generation biorefineries. myo-Inositol, as vitamin B8, has a six-carbon carbon-carbon ring. Here we designed an in vitro artificial NAD(P)-free 12-enzyme pathway that can effectively convert the five-carbon xylose to inositol involving xylose phosphorylation, carbon-carbon (C-C) rearrangement, C-C bond circulation, and dephosphorylation. The reaction conditions catalyzed by all thermostable enzymes from hyperthermophilic microorganisms Thermus thermophiles, Thermotoga maritima, and Archaeoglobus fulgidus were optimized in reaction temperature, buffer type and concentration, enzyme composition, Mg2+ concentration, and fed-batch addition of ATP. The 11-enzyme cocktail, whereas a fructose 1,6-bisphosphatase from T. maritima has another function of inositol monophosphatase, converted 20 mM xylose to 16.1 mM inositol with a conversion efficiency of 96.6% at 70 °C. Polyphosphate was found to replace ATP for xylulose phosphorylation due to broad substrate promiscuity of the T. maritima xylulokinase. The Tris-HCl buffer effectively mitigated the Maillard reaction at 70 °C or higher temperature. The co-production of value-added biochemicals, such as inositol, from wood sugar could greatly improve economics of new biorefineries, similar to oil refineries that make value-added plastic precursors to subsidize gasoline/diesel production.

Aims and Scope (i) Why do we feel that the issue is important and timely? Research in renewable biopolymers as substitutes for full-carbon-backbone plastics from fossil resources presents a topical R&D field worldwide. This is due to the ongoing depletion of fossil resources, the growing piles of plastic waste and plastic pollution of marine environments, and the need to convert waste streams of different industrial origin in a value-added way. PHA have the potential to replace established petro-plastics both in bulk applications as packaging material and in niche applications, such as the medical, electronical, etc. field. Moreover, the close relation of PHA production and application to the food sector becomes more and more evident. Not only does food production provide numerous (ago) industrial by-products which can, on the one hand, be applied to boost growth kinetics of PHA-accumulation strains, as evidenced in the case of nitrogenaceous whey retentate, silage residues, or shrimps waste, and, on the other hand, act as feedstocks for PHA-biosynthesis under nutritionally unbalanced growth conditions, as demonstrated for carbonaceous surplus materials like whey permeate, lignocellulosics, glycerol, waste lipids, etc. Moreover, PHA are currently investigated as future materials contributing to safe and smart food storage and packaging, as shown by PHA´s beneficial gas barrier properties. Grace to the high compatibility of PHA with numerous organic and inorganic additives, a range of promising PHA-based blend of composite materials are accessible to design novel food packaging materials. This encompasses the application of lignocellulosic filler materials from rice, sugar, or wood production, and even the development of more sophisticated formulations resorting to the incorporation of functional nanoparticles into PHA matrixes. (ii) What communities are expected to participate in the Special Issue? Scientific community; Scholars of higher level; Industrialists (iii) How are the background and expertise of the authors relevant to the proposed Special Issue? List of topics for the Special Issue.