## 1 Introduction  
Platform chemicals are molecules that can be used to make a variety of industrially valuable products including solvents, polymers, pharmaceuticals and foods [1]. Most commercial platform chemicals are mass-produced from petrochemical processes that use fossil fuels as the starting raw materials [2], typically via low-cost, high-volume commodity chemicals (methanol, ethylene, propene, butadiene, benzene, toluene and xylene) [3]. While recent advances have enabled the use of renewable approaches on an industrial scale (such as the production of polylactide and polyethylene from ethylene and biomass-derived lactic acid [4]), the sustainable production of a wider range of platform chemicals is a priority given global concerns about the environment, limited resources, and high oil prices [3, 5]. One platform chemical in high global demand is malic acid. Malic acid, along with other four-carbon diacids (such as succinic and fumaric acids), was reported as one of the top value-added chemicals from biomass in a 2004 U.S. Department of Energy report [6]. Four-carbon diacids can be converted into 1,4-butanediol which can be further converted into high-value products such as tetrahydrofuran, an important material for polymer production. The current commercial production of malic acid is via the chemical hydration of petrochemically derived maleic/fumaric acid [7], but advances in the field of metabolic engineering have renewed interest in its bio-based production [8]. Natural high-capacity malic acid producers such as _Aspectzillus flavus_ are not well-suited to industrial use due to their physiological constraints [3] and the associated generation of aflatoxin (a carcinogen). A particular concern is the need to control multiple metabolic pathways that lead to the formation of unwanted intermediates [9], thereby reducing conversion efficiencies. Cell-free metabolic engineering uses purified enzyme systems or crude cell lysates in the synthesis of complex biomolecular commodities, thereby overcoming limitations associated with using whole cells [3]. However, the high cost of processes involving enzyme purification and the need to supplement reactions with the necessary cofactors are current challenges. The nicotinamide nucleotides NAD\({}^{+}\) and NADP\({}^{+}\) are used by most redox enzymes, and their balance is crucial in cell-free enzymatic pathways which lack metabolic regulation from living cells [3]. Developing cascades consisting of multiple enzymes in cell-free pathways is a recent development [10]. This has led to developments in the design of cell-free systems in which cofactors can be recycled on account of their high costs [11]. Glucose dehydrogenase (GDH), along with formate dehydrogenase (FDH), are the two most widely used enzymes for cofactor recycling in industrial/commercial processes, with GDH from the _Bacillus_ species showing the highest activity and stability, leading to its wide applications in industry [12]. NADP(H)-dependent malate dehydrogenase from the thermophilic archaeon _Thermo-coccus kodakrenensis_, catalyzes the HCO\({}_{3}\)\({}^{-}\) independent reductive carboxylation of pyruvate to malate and produces lactate as a by-product. Its application in artificial enzymatic cascades that use NADH as a cofactor is now possible following directed evolution to alter its cofactor preference from NADPH to NADH [13]. A triple mutant was identified that exhibited a sixfold higher \(\mathrm{k_{cat}}/\mathrm{K_{M}}\) for NAD\({}^{+}\) than the wildtype enzyme together with increased malate (and lower lactate) yields. The triple mutant enzyme (hereafter referred to as MDH*) catalyzed pyruvate carboxylation with NADH to give a 1.2 times higher yield of malate than the wildtype enzyme with NADPH [13]. We previously immobilized purified recombinant GDH from _Sulfolobus solfataricus_ onto novel, hierarchically structured macroporous-mesoporous silica supports and produced gluconic acid from both glucose and bread waste hydrolysate using NAD\({}^{+}\) as a cofactor.