Nonribosomal peptide synthetase; part of the gene cluster that mediates the biosynthesis of butyrolactones, natural products that show a wide range of biological activities such as antitumor, antiparasitic or anti-inflammatory activity (PubMed:29305695, PubMed:23841722, PubMed:27978657). The nonribosomal peptide synthetase btyA is responsible for the production of butyrolactone II, the core structure of butyrolactones (PubMed:29305695, PubMed:27978657). BtyA first activates 4-hydroxyphenylpyruvate (HPPA) through its A domain to AMP-HPPA (Probable). The HPPA unit is then loaded to the T domain and eventually transferred to the TE domain (Probable). Upon loading of another HPPA unit to the T domain, the TE domain promotes the enolate formation on the unit attached (Probable). Then aldol condensation establishes the carbon-carbon bond between the two units, followed by ester cyclization, and keto-enol tautomerization to yield the gamma-butyrolactone core (Probable). Hydrolysis, and finally esterification of the exposed carboxylic acid group yields butyrolactone II (Probable). Two additional enzymes, a prenyltransferase and an epoxidase, may be involved in the tailoring modifications of butyrolactone II to give butyrolactone III and butyrolactone I (Probable). 2 3-(4-hydroxyphenyl)pyruvate + H(+) = (2S)-2-(4-hydoxybenzyl)-3-(4-hydroxyphenyl)-2-furonol carboxylate + H2O Secondary metabolite biosynthesis. BtyA has an A-T-TE domain architecture (Probable). The adenylation (A) domain recognizes and activates the aryl acid substrates, and loads them onto the thiolation (T) domain (Probable). The thioesterase (TE) domain shares the missing condensation (C) domain function, and is responsible for condensation and final product release (Probable). Belongs to the NRP synthetase family.