초록
<P>Trihydroxyphenolic acids such as 3,4,5-trihydroxycinnamic acid (3,4,5-THCA) <B>4c</B> and 2-(3,4,5-trihydroxyphenyl)acetic acid (3,4,5-THPA) <B>2c</B> are strong antioxidants that are potentially useful as medicinal agents. Our results show that <I>p</I>-hydroxyphenylacetate (HPA) 3-hydroxylase (HPAH) from <I>Acinetobacter baumannii</I> can catalyze the syntheses of 3,4,5-THPA <B>2c</B> and 3,4,5-THCA <B>4c</B> from 4-HPA <B>2a</B> and <I>p</I>-coumaric acid <B>4a</B>, respectively. The wild-type HPAH can convert 4-HPA <B>2a</B> completely into 3,4,5-THPA <B>2c</B> within 100 min (total turnover number (TTN) of 100). However, the wild-type enzyme cannot efficiently synthesize 3,4,5-THCA <B>4c</B>. To improve the efficiency, the oxygenase component of HPAH (C<SUB>2</SUB>) was rationally engineered in order to maximize the conversion of <I>p</I>-coumaric acid <B>4a</B> to 3,4,5-THCA <B>4c</B>. Results from site-directed mutagenesis studies showed that Y398S is significantly more effective than the wild-type enzyme for the synthesis of 3,4,5-THCA <B>4c</B>; it can catalyze the complete bioconversion of <I>p</I>-coumaric acid <B>4a</B> to 3,4,5-THCA <B>4c</B> within 180 min (TTN ∼ 23 at 180 min). The yield and stability of 3,4,5-THPA <B>2c</B> and 3,4,5-THCA <B>4c</B> were significantly improved in the presence of ascorbic acid. Thermostability studies showed that the wild-type C<SUB>2</SUB> was very stable and remained active after incubation at 30, 35, and 40 °C for 24 h. Y398S was moderately stable because its activity was retained for 24 h at 30 °C and for 15 h at 35 °C. Transient kinetic studies using stopped-flow spectrophotometry indicated that the key improvement in the reaction of Y398S with <I>p</I>-coumaric acid <B>4a</B> lies within the protein–ligand interaction. Y398S binds to <I>p</I>-coumaric acid <B>4a</B> with higher affinity than the wild-type enzyme, resulting in a shift in equilibrium toward favoring the productive coupling path instead of the path leading to wasteful flavin oxidation.</P><P><B>Graphic Abstract</B><BR><IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/accacs/2015/accacs.2015.5.issue-8/acscatal.5b00439/production/images/medium/cs-2015-00439u_0012.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cs5b00439'>ACS Electronic Supporting Info</A></P>