|
type |
Journal Article |
authors |
Crnovcić I, Süssmuth R, Keller U. |
title |
Aromatic C-methyltransferases with antipodal stereoselectivity for structurally diverse phenolic amino acids catalyze the methylation step in the biosynthesis of the actinomycin chromophore. |
journal |
Biochemistry. |
Activity |
acmh |
Family |
acmh |
PLP Fold Type |
1 |
sel |
selected |
ui |
20945860 |
year |
(2010) |
volume |
49 |
number |
45 |
pages |
9698-705 |
| |
abstract |
The actinomycin biosynthetic gene cluster of Streptomyces chrysomallus harbors two paralogous genes, acmI and acmL, encoding methyltransferases. To unveil their suspected role in the formation of 3-hydroxy-4-methyl-anthranilic acid (4-MHA), the building block of the actinomycin chromophore, each gene was expressed in Escherichia coli. Testing the resulting ∼40 kDa His(6)-tagged proteins with compounds of biogenetic relevance as substrates and S-adenosyl-l-methionine revealed that each exclusively methylated 3-hydroxykynurenine (3-HK) with formation of 3-hydroxy-4-methylkynurenine (4-MHK) identified by its in vitro conversion to 4-MHA with hydroxykynureninase. AcmI and AcmL methylate also hydroxyphenyl-amino propanoic acids such as p-tyrosine, m-tyrosine, or 3,4-dihydroxy-l-phenylalanine (DOPA) but at a lower rate than 3-HK. The presence of the α-amino group was necessary for substrate recognition. Phenolic acids with shorter chains such as 4-hydoxyphenyl-l-glycine (HPG), 3-hydroxybenzoic acid (3-HB), or 3-hydroxyanthranilic acid (3-HA) gave no product. Both enzymes were stereospecific for the optical configuration at α-C with unprecedented antipodal selectivity for the d-enantiomer of 3-HK and the l-enantiomer of p-tyrosine or m-tyrosine. AcmI and AcmL show sequence similarity to various C- and O-methyltransferases from bacteria. Phylogenetic analysis places them into the clade of C-methyltransferases comprising among others orthologues involved in 4-MHA formation of other biosynthesis systems and methyltransferases putatively involved in the C-methylation of tyrosine. Remarkably, computational remodelling of AcmI and AcmL structures revealed significant similarity with the 3-D structures of type 1 O-methyltransferases from plants such as caffeic acid O-methyltransferase (COMT) and other phenylpropanoid methyltransferases. The relevance of 3-HK or 3-HA methylation in the actinomycin biosynthesis pathways of different actinomycetes is discussed.
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last changed |
2018/04/18 16:02 |
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