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 derivatives, including the branched-chain aminotransferase (BCAT), branched-chain keto-acid dehydrogenase (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 4-hydroxylase (C4H), chalcone synthase (CHS_H1), chalcone isomerase (CHI)-like proteins (CHIL), and O-methyltransferase (OMT1). Furthermore, research advancements of each enzyme in terms of reaction conditions, substrate recognition, enzyme structures, and use in engineered microbes are described in depth. Hence, an extensive review of the key enzymes involved in the phytochemical compounds of hops will provide fundamentals for their applications in beer production.
Highlights
Female hop cone inflorescences are frequently employed in the brewing industry
Hops’ brewing value is primarily attributable to the precursors of flavor- and bitter-active compounds found in lupulin gland resins
Branched-chain aminotransferase (BCAT), branched-chain keto-acid dehydrogenase (BCKDH), carboxyl CoA ligase (CCL), valerophenone synthase (VPS), and prenyltransferase (PT) are the major enzymes involved in the production of CoAs and DMAPP pathways
Summary
Female hop cone inflorescences are frequently employed in the brewing industry. Around a strig (or central rachis) are petal-like structures on the hop cone called ‘bracts’ and ‘bracteoles’. This section biosynthesizes specific secondary metabolites, such as hop essential oil, hop polyphenols, and total resins, including hop bitter acid (α- and β-acids) and prenylated flavonoids, the xanthohumol derivatives. In-depth studies of the properties of the various phytochemical compounds have encouraged researchers to optimize their use in the beer brewing process. This has prompted individuals to focus on getting more of these essential compounds during upstream growth and preparing them via other techniques. The rate-limiting enzymes in the biosynthesis pathway of hop phytochemical substances contributing to bitter flavor, aroma, and health were evaluated based on their contributions to beer quality. The crystal structure of several rate-limiting enzymes needs to be further resolved, further development of which will help synthesize the phytochemical compounds in vitro
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