International Journal of Molecular Sciences Review Key Enzymes Involved in the Synthesis of Hops Phytochemical Compounds: From Structure, Functions to Applications Kai Hong , Limin Wang, Agbaka Johnpaul , Chenyan Lv * and Changwei Ma * College of Food Science and Nutritional Engineering, China Agricultural University, 17 Qinghua Donglu Road, Haidian District, Beijing 100083, China;
[email protected] (K.H.);
[email protected] (L.W.);
[email protected] (A.J.) * Correspondence:
[email protected] (C.L.);
[email protected] (C.M.); Tel./Fax: +86-10-62737643 (C.M.) Abstract: Humulus lupulus L. is an essential source of aroma compounds, hop bitter acids, and xanthohumol derivatives mainly exploited as flavourings in beer brewing and with demonstrated potential for the treatment of certain diseases. To acquire a comprehensive understanding of the biosynthesis of these compounds, the primary enzymes involved in the three major pathways of hops’ phytochemical composition are herein critically summarized. Hops’ phytochemical components impart bitterness, aroma, and antioxidant activity to beers. The biosynthesis pathways have been extensively studied and enzymes play essential roles in the processes. Here, we introduced the enzymes involved in the biosynthesis of hop bitter acids, monoterpenes and xanthohumol deriva- tives, including the branched-chain aminotransferase (BCAT), branched-chain keto-acid dehydroge- nase (BCKDH), carboxyl CoA ligase (CCL), valerophenone synthase (VPS), prenyltransferase (PT), 1-deoxyxylulose-5-phosphate synthase (DXS), 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR), Geranyl diphosphate synthase (GPPS), monoterpene synthase enzymes (MTS), cinnamate Citation: Hong, K.; Wang, L.; 4-hydroxylase (C4H), chalcone synthase (CHS_H1), chalcone isomerase (CHI)-like proteins (CHIL), Johnpaul, A.; Lv, C.; Ma, C.