Kota Parvathi, Guo Dianjing, Zubieta Chloe, Noel Joe, Dixon Richard A
Plant Biology Division, Samuel Roberts Noble Foundation, 2510 Sam Noble Parkway, Ardmore, OK 73401, USA.
Phytochemistry. 2004 Apr;65(7):837-46. doi: 10.1016/j.phytochem.2004.01.017.
Although S-adenosyl-l-methionine (SAM) dependent caffeic acid/5-hydroxyferulic acid 3/5-O-methyltransferase (COMT) is one of the key enzymes in lignin biosynthesis, the present work demonstrates that alfalfa COMT methylates benzaldehyde derivatives more efficiently than lignin pathway intermediates. 3,4-Dihydroxy, 5-methoxybenzaldehyde and protocatechuic aldehyde were the best in vitro substrates for OMT activity in extracts from developing alfalfa stems, and these compounds were preferred over lignin pathway intermediates for 3-O-methylation by recombinant alfalfa COMT expressed in Escherichia coli. OMT activity with benzaldehydes was strongly reduced in extracts from stems of transgenic alfalfa down-regulated in COMT. However, although COMT down-regulation drastically affects lignin composition, it does not appear to significantly impact metabolism of benzaldehyde derivatives in alfalfa. Structurally designed site-directed mutants of COMT showed altered relative substrate preferences for lignin precursors and benzaldehyde derivatives. Taken together, these results indicate that COMT may have more than one role in phenylpropanoid metabolism (but probably not in alfalfa), and that engineered COMT enzymes could be useful for metabolic engineering of both lignin and benzaldehyde-derived flavors and fragrances.
尽管依赖S-腺苷-L-甲硫氨酸（SAM）的咖啡酸/5-羟基阿魏酸3/5-O-甲基转移酶（COMT）是木质素生物合成中的关键酶之一，但目前的研究表明，苜蓿COMT对苯甲醛衍生物的甲基化效率高于木质素途径中间体。3,4-二羟基-5-甲氧基苯甲醛和原儿茶醛是发育中的苜蓿茎提取物中OMT活性的最佳体外底物，在大肠杆菌中表达的重组苜蓿COMT对这些化合物进行3-O-甲基化时，比木质素途径中间体更受青睐。在COMT表达下调的转基因苜蓿茎提取物中，苯甲醛的OMT活性显著降低。然而，尽管COMT下调会极大地影响木质素组成，但似乎对苜蓿中苯甲醛衍生物的代谢没有显著影响。结构设计的COMT定点突变体对木质素前体和苯甲醛衍生物的相对底物偏好发生了改变。综上所述，这些结果表明COMT在苯丙烷代谢中可能具有多种作用（但可能在苜蓿中并非如此），并且工程化的COMT酶可能对木质素以及苯甲醛衍生的风味和香料的代谢工程有用。
