Abstract
Betalains are a group of nitrogen-containing pigments that color plants in most families of Caryophyllales. Their biosynthesis has long been proposed to begin with hydroxylation of L-tyrosine to L-DOPA through monophenolase activity of tyrosinase, but biochemical evidence in vivo remains lacking. Here we report that a Group 4 catalase, catalase-phenol oxidase (named as AcCATPO), was identified, purified and characterized from leaves of Amaranthus cruentus, a betalain plant. The purified enzyme appeared to be a homotrimeric protein composed of subunits of about 58 kDa, and demonstrated not only the catalase activity toward H2O2, but also the monophenolase activity toward L-tyrosine and diphenolase activity toward L-DOPA. Its catalase and phenol oxidase activities were inhibited by common classic catalase and tyrosinase inhibitors, respectively. All its peptide fragments identified by nano-LC-MS/MS were targeted to catalases, and matched with a cDNA-encoded polypeptide which contains both classic catalase and phenol oxidase active sites. These sites were also present in catalases of non-betalain plants analyzed. AcCATPO transcript abundance was positively correlated with the ratio of betaxanthin to betacyanin in both green and red leaf sectors of A. tricolor. These data shows that the fourth group catalase, catalase-phenol oxidase, is present in plant, and might be involved in betaxanthin biosynthesis.
Highlights
Betalains are vacuole-localized, water-soluble, nitrogen-containing pigments, which consist of red-violet betacyanins and yellow-orange betaxanthins (Gandía-Herrero et al, 2005; Hatlestad et al, 2012)
The results showed that both conserved catalase active site and potential oxidase active site were present in the catalases of both betalain plants and non-betalain plants, and obvious difference between two group plants lay in the proportion of triple amino acids (PTM) used as peroxisomal targeting signal: 90% in the catalases of betalain plants and 16.7% in those of non-betalain plants (Table 3 and Supplementary Data 5)
The AcCATPO possessed the monophenolase activity toward L-tyrosine and diphenolase activity to L-DOPA besides the catalase activity, so was it involved in betalain biosynthesis? With this question in mind, we examined the relation between the betalain content and the expression level of AcCATPO in green and red leaf sectors of A. tricolor
Summary
Betalains are vacuole-localized, water-soluble, nitrogen-containing pigments, which consist of red-violet betacyanins and yellow-orange betaxanthins (Gandía-Herrero et al, 2005; Hatlestad et al, 2012). The classical tyrosinase is considered to be a bifunctional polyphenol oxidase (PPO) (Strack and Schliemann, 2001) It catalyzes two distinct and continuous reactions in the presence of molecular oxygen: the hydroxylation of monophenols to o-diphenols (EC 1.14.18.1; tyrosine hydroxylase activity or monophenolase activity) and their subsequent oxidation to the corresponding o-quinones (EC 1.10.3.1; diphenolase activity). The monophenolase activity is a key feature for distinguishing tyrosinases involved in biosynthesis of specialized metabolites such as betalains from other PPOs associated with plant browning and defense (Strack and Schliemann, 2001; Sullivan, 2015)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
