| Significant progress has been made in lignin biodegradation research since 1983, when lignin peroxidases were discovered in fungi. A similar breakthrough in bacterial lignin biodegradation research is anticipated. Several laboratories have successfully demonstrated the ability of bacteria to mineralize {dollar}lbracksp{lcub}14{rcub}{dollar}C-lignin{dollar}rbrack{dollar}lignocelluloses as well as {dollar}sp{lcub}14{rcub}{dollar}C-labelled synthetic lignins. Attempts are being made to identify the key enzymes involved. In this dissertation, two scientific papers are presented which address the enzymology of lignin biodegradation by Streptomyces viridosporus. The first manuscript describes a comparative study of selected extracellular enzymes of wild-type and genetically manipulated strains with enhanced abilities to produce a water soluble lignin degradation intermediate, designated acid-precipitable polymeric lignin (APPL). UV irradiation mutant T7A-81 and protoplast fusion recombinant SR-10 had higher and longer persisting peroxidase, esterase, and endoglucanase activity than did the wild type strain T7A. Results implicated one or more of these enzymes in lignin bioconversion. Only mutant T7A-81 had higher xylanase activity than the wild type. An extracellular lignocellulose-induced peroxidase with some similarities to fungal ligninases was described for the first time in Streptomyces. Peroxidase and esterase isoenzymes differed in their substrate specificities, though there were no major differences in the isoenzymes produced by the different strains. Endoglucanases also exhibited xylanase activity. The second manuscript describes purification and characterization of an extracellular lignin peroxidase produced by S. viridosporus T7A. This is the first report of a lignin peroxidase in any bacterium. Purified peroxidase preparation with high isoenzyme P3 activity was tested for its ability to oxidize lignin model compounds of both the 1,2-diarylpropane and arylglycerol-{dollar}beta{dollar}-aryl ether types containing C{dollar}sb{lcub}alpha{rcub}{dollar}-carbonyl and C{dollar}sb{lcub}alpha{rcub}{dollar}-hydroxyl groups. In the presence of added hydrogen peroxide the enzyme catalyzed C{dollar}sb{lcub}alpha{rcub}{dollar}-C{dollar}sb{lcub}beta{rcub}{dollar} bond cleavage in the side chains of the diaryl ethers, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol, 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propan-1-one (II) and a diaryl ethane, 1-(4-methoxyphenyl)-2-(phenyl)ehan-1-one. These results suggested that this is a major enzyme involved in lignin biodegradation, since these compounds together represent the major intermonomeric linkages in the lignin polymer. Results also indicated that enzyme oxidized native lignocellulosic substrates. Additional characterizations showed that the enzyme is a heme protein. |
