Tukayi Kudanga

Laccase catalysis for the synthesis of bioactive compounds

The demand for compounds of therapeutic value is increasing mainly because of new applications of bioactive compounds in medicine, pharmaceutical, agricultural, and food industries. This has necessitated the search for cost-effective methods for producing bioactive compounds and therefore the intensification of the search for enzymatic approaches in organic synthesis. Laccase is one of the enzymes that have shown encouraging potential as biocatalysts in the synthesis of bioactive compounds. Laccases are multicopper oxidases with a diverse range of catalytic activities revolving around synthesis and degradative reactions. They have attracted much attention as potential industrial catalysts in organic synthesis mainly because they are essentially green catalysts with a diverse substrate range. Their reaction only requires molecular oxygen and releases water as the only by-product. Laccase catalysis involves the abstraction of a single electron from their substrates to produce reactive radicals. The free radicals subsequently undergo homo- and hetero-coupling to form dimeric, oligomeric, polymeric, or cross-coupling products which have practical implications in organic synthesis. Consequently, there is a growing body of research focused on the synthetic applications of laccases such as organic synthesis, hair and textile dyeing, polymer synthesis, and grafting processes. This paper reviews the major advances in laccase-mediated synthesis of bioactive compounds, the mechanisms of enzymatic coupling, structure-activity relationships of synthesized compounds, and the challenges that might guide future research directions.

Secretory expression of recombinant small laccase from Streptomyces coelicolor A3(2) in Pichia pastoris

This work reports for the first time the secretory expression of the small laccase (SLAC) from Streptomyces coelicolor A3(2) in Pichia pastoris. Using an AOX1 promoter and α factor as a secretion signal, the recombinant P. pastoris harbouring the laccase gene (rSLAC) produced high titres of extracellular laccase (500 ± 10 U/l), which were further increased seven fold by pre-incubation at 80 °C for 30 min. The enzyme (∼38 kDa) had an optimum activity at 80 °C, but optimum pH varied with substrate used. Km values for ABTS, SGZ and 2,6-DMP were 142.85 μM, 10 μM and 54.55 μM and the corresponding kcat values were 60.6 s-1, 25.36 s-1 and 27.84 s-1, respectively. The t1/2 values of the rSLAC at 60 °C, 70 °C, 80 °C were 60 h, 32 h and 10 h, respectively. The enzyme deactivation energy (Ed) was 117.275 kJ/mol while ΔG, ΔH and ΔS for thermal inactivation of the rSLAC were all positive. The rSLAC decolourised more than 90% of Brilliant Blue G and Trypan Blue dye in 6 h without the addition of a mediator. High titres of SLAC expressed in P. pastoris enhance its potential for various industrial applications.

Opuntia (Cactaceae) plant compounds, biological activities and prospects – A comprehensive review

Opuntia species are utilized as local medicinal interventions for chronic diseases and as food sources mainly because they possess nutritional properties and biological activities. The Opuntia plant is distributed worldwide and has great economic potential. Differences in Opuntia species phytochemical composition exist between wild and domesticated species, and within species. Opuntia aerial and underground parts exhibit beneficial properties due to their phenolic content, other antioxidants (for example ascorbate), pigments (carotenoids, betalains), and other unidentified components. This work comprehensively reviews the phytochemical composition of the different aerial and underground plant parts of Opuntia species. The applications of Opuntia compounds and their biological activities are also discussed. Other topical aspects covered include Opuntia spp. taurine composition, Opuntia side effects, Opuntia by-products valorisation and the role of Opuntia spp. in tackling antimicrobial resistance. Although biological activities have been extensively researched, much less information is available on reaction mechanisms, herbal mixtures toxicology and commercialisation prospects - aspects which should be considered for future research in this area.

Composition, thermal and rheological properties of polysaccharides from amadumbe ( Colocasia esculenta ) and cactus ( Opuntia spp.)

The extensive application of hydrocolloids in the food industry, coupled with their short supply and shortcomings, has led to the ongoing search for alternative sources. In this study, the compositional, rheological and thermal properties of amadumbe and cactus mucilages were investigated. The mucilages had a similar qualitative composition of monosaccharides and amino acids, except for the absence of rhamnose in amadumbe mucilage. Fractionation of amadumbe and cactus mucilages on an anion-exchange column yielded four and three fractions, respectively. The fractions eluting with protein showed no β-elimination, suggesting stronger glycosylation bonds such as those in arabinogalactan proteins (AGPs). There was no evidence of thermal depolymerisation of the mucilages up to 195 °C. Cactus mucilage showed a pseudoplastic flow behaviour whilst amadumbe mucilage showed a Newtonian flow behaviour at up to 5% (w/v) concentrations. Amadumbe mucilage may be a potential emulsifier, whilst cactus mucilage can potentially be used as a thickening or emulsifying agent.