Review 1 DOI: 10.1002/ijch.201800167 2 3 4 Heme-Derived Bilins 5 Jon Y. Takemoto,*[a] Cheng-Wei T. Chang,[b] Dong Chen,[c] and Garrett Hinton[d] 6 7 8 9 Abstract: Living things on Earth depend on heme – the iron- ingly appreciated for their diverse roles such as sensing and 10 cyclic tetrapyrrole complex that harnesses iron’s oxidizing gathering light, regulating growth and aging, responding to 11 powers. Heme is toxic, but Nature has evolved ways to inflammatory conditions, and influencing behavior. The 12 control it. One way is breaking it with heme oxygenase which diverse functions of bilins are exploited with discoveries and 13 lowers its levels and begins the formation of linear uses of bioactive bilins for salutary benefits in medicine and 14 tetrapyrroles called bilins. Bilins occur in many variations, agriculture. Opportunities for finding new bioactive bilins 15 often colorful, sometimes in abundance, and in animals, and applications will grow as knowledge of bilin biology and 16 plants and microbes. Contrary to early notions, bilins are not capabilities for producing bilins continue to expand. 17 only waste products of heme degradation. They are increas- 18 Keywords: antioxidants · bilins · biliverdin · heme · mesobiliverdin 19 20 21 1. Introduction. The Heme Enigma and Bilins focus on currently known bioactive bilins and future directions 22 for bilin discovery and applications as useful natural products. 23 The Fe-cyclic tetrapyrrole complex, heme (Fe-protoporphyrin 24 IX) (1, Figure 1) is ubiquitous in Nature. 25 It emerged early in evolution to exploit Earth’s second 2. Heme Oxygenase and Pathways toward Bilins 26 most abundant metal, Fe.[1] Heme’s functions are beneficial 27 and critical to life. But heme is also toxic[2] and therefore an All known heme degradation systems begin with cleavage of 28 enigmatic problem for life. Many of heme’s beneficial one of four heme methene bridges by heme oxygenase (EC 29 functions are conferred when bound to apoproteins,[3] and a 1.14.99.3) (Figure 2). The enzymatic products after three 30 variety of hemeprotein complexes catalyze critical physiolog- successive oxygenation steps are linear tetrapyrroles (biliver- 31 ical functions. Among them are gas exchange, storage and dins), CO and Fe. Biliverdin isomers (2-5, Figure 1) are 32 transport as performed by hemoglobin, leghemoglobin and produced depending on whether the a, b, g or d methene 33 myoglobin. Also, hemeproteins function in electron transport, bridge is cleaved by heme oxygenase (Figure 2).[4] The most 34 oxidation reduction, drug metabolism and detoxification. In common variation is cleavage at the a position by canonical 35 contrast, free heme Fe initiates Fenton chemistry to create heme oxygenase-1 (HO-1) which occurs in animals, higher 36 reactive oxygen species that inflict damage to cell lipids, plants, algae, fungi and bacteria to give the bilin, biliverdin (2, 37 proteins, and nucleic acids.[2] Nature has evolved ways to deal Figure 1). 38 with heme toxicity. Mammalian animals degrade free heme 39 and excrete a large portion of the catabolic metabolites to 40 lower its concentration. Other animals, plants, and microbes 41 utilize heme for growth and survival or carefully regulate the 42 levels of biosynthetic heme to below toxic levels. In all cases, 43 linear t etrapyrroles called “bilins” (Figure 1) that are end 44 products of heme catabolism are generated. The bilins are [a] J. Y. Takemoto 45 typically pigmented, lack toxicity, and often accumulate in Department of Biology, Utah State University, Logan, Utah 84322- 5305 U.S.A 46 abundance. Telephone: 1–435-7970671 47 The chemistry and biology of bilins have long been Fax: 1–435-7971575 48 popular research subjects. In addition to being heme catabolic E-mail: [email protected] 49 waste products in mammals, well-known roles include photo- [b] C.-W. T. Chang 50 morphogenesis, energy harvesting and transfer and effects on Department of Chemistry and Biochemistry, Utah State University, 51 animal behavior. More recently, it has become evident that Logan, Utah 84322-0300, U.S.A 52 certain bilins can be bioactive with beneficial physiological [c] D. Chen Department of Biological Engineering, Utah State University, Lo- 53 effects. Future discoveries of the bioactivities of bilins are gan, Utah 843122, U.S.A 54 anticipated. This mini-review attempts to briefly introduce the [d] G. Hinton 55 biological and chemical scope of heme-derived bilins and then Department of Biology, Utah State University, Logan, Utah 84322- 56 5305 U.S.A Isr. J. Chem. 2019, 59, 378 –386 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 378 Review 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Figure 1. Structures of heme and heme-derived bilins discussed. 1, heme (Fe protoporphyrin IX); 2, biliverdin (biliverdin IXa); 3, biliverdin IXb; 27 4, biliverdin IXd; 5, biliverdin IXg; 6, bilirubin (bilirubin IXa); 7, urobilin (urobilin IXa); 8, urobilinogen (urobilinogen IXa); 9, stercobilin 28 (stercobilin IXa); 10, phycoerythrobilin; 11, phycocyanobilin; 12,181,182-dihydro-biliverdin IX; 13 phycouribilin; 14, phycoviolobilin;15, 29 phytochromobilin; 16, mesobiliverdin (mesobiliverdin IXa); 17,15,16-dihydro-biliverdin IXa; 18, bilirubin diglucuronide. 30 31 32 33 34 Prof. Jon Y. Takemoto received B.A. and Ph.D. of Chemistry and Biochemistry, Utah State 35 (1973) degrees in microbiology from the Uni- University since 2000. versity of California, Los Angeles, and was a 36 Dr. Dong Chen received his B.S. degree in 1982 postdoctoral fellow at the Biological Laborato- 37 from Xiamen University and Ph. D. degree in ries, Harvard University (1973–74). He has 1988 in Bioinorganic Chemistry from Nanjing 38 been on the Biology Department faculty at University, P. R. China. He was a Postdoctoral 39 Utah State University and the Utah Agricultural Fellow in Bioinorganic Chemistry in Dr. Helmut 40 Experiment Station since 1975. His research Sigel’s group at Basel University, Switzerland 41 topics are physiology of photosynthetic mi- (1991-93). He has worked at Utah State crobes, biology and chemistry of bacterial cyclic 42 University since 1993. His main research lipodepsipeptides, and microbial natural prod- 43 interests are protein modeling, identification ucts discovery. His has mentored 27 graduate 44 and expression, microbial fermentation, and students and a dozen postdoctoral associates. 45 the discovery of novel microbial natural prod- 46 Prof. Tom Chang received his B.S. degree ucts. (organic chemistry) in 1988 from Tunghai 47 Garrett Hinton is currently a Ph.D. student at University (Taiwan) and a Ph.D. degree from the University of Arizona in the College of 48 Washington University (St. Louis, MO.) in Optical Sciences. He received a B.S. degree in 49 1997. Trained as a synthetic chemist, his Electrical Engineering from Utah State Univer- 50 research career encompasses chemistry, biol- sity in 2017 where he pursued research in 51 ogy, and microbiology. He has pursued re- biology. His research work involved bilin bio- search in antimicrobial and anticancer areas 52 chemistry and cell biology – particularly imag- focusing on product development and basic 53 ing bilin action in mammalian cells which science. Newer research areas are alternative 54 inspired him to go into the field of optics. energy development and green chemistry. He 55 has been a faculty member of the Department 56 Isr. J. Chem. 2019, 59, 378 –386 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.ijc.wiley-vch.de 379 Review 1 capable of crossing the placental barrier contributing further to 2 bilirubin accumulation (hyperbilirubinimea or jaundice) in the 3 fetus and neonates.[4b,7] Such accumulation is speculated to 4 have a protective role against streptococcal sepsis due to the 5 antibacterial action of the bilirubins.[8] Interestingly, BVRB is 6 also expressed in hepatocellular carcinomas and prostate 7 cancer cells.[9] 8 9 10 2.2 Non-Mammalian Animal Bilins 11 12 Many non-mammalian animals accumulate heme-derived 13 bilins that impart color, and a large fraction are biliverdins, 14 mostly the biliverdin IXa form.[10] The bluish or greenish 15 colors of avian and dinosaur egg shells,[10–11] fish,[12] and 16 lizards,[13] and frogs[14] are attributed to biliverdin. Other 17 biliverdin isomers and bilins occur in insects,[15] helminths,[16] 18 and marine snails.[17] In response to threats, bilins described as 19 biliverdins rapidly accumulate in circulating blood of skinks, 20 Figure 2. Heme and biliverdin isomer products after methene bridge veiled chameleons[18] and certain frogs.[14] The reasons for 21 cleavage by HO-1 at the a, b, g and d positions. The bold numerals rapid bilin appearance in these small animals are debated[11] 22 refer to the numbered bilins in Figure 1. and range widely from a defense mechanism against predator 23 attack to innate immunity.[13,19] Despite the ability of many 24 non-mammalian animals to accumulate biliverdin, BVRA still 25 2.1 Mammalian Animal Bilins occurs as evidence by the occurrence of bilirubin in these 26 animals.[20] 27 In animals, biliverdin comprises >90% of the bilins generated 28 by HO-1 and HO-2. In red-blooded animals, biliverdin is 29 rapidly reduced via biliverdin reductase A (BVRA) (EC 2.3 Plant, Cyanobacterial, and Algal Bilins 30 1.3.1.24) in the liver to bilirubin (6) which may be toxic when 31 accumulated (although this notion is disputed.[4b] Liver In the photosynthetic organisms, plants, cyanobacteria, and 32 bilirubin is glycosylated with glucuronic acid by uridine 5’- algae, HO-generated biliverdin is biosynthetically converted 33 diphosphate glucuronosyl transferase (UGT) (E.C. 2.4.1.17) to via biliverdin reductases to pigmented bilins that sense and 34 make a more hydrophilic conjugated form, bilirubin diglucur- transmit light energy.[21] In the model plant Arabidopsis 35 onide (18, Figure 1) for easier excretion in bile and urine thaliana, 4 HO’s are expressed with the majority of HO 36 (Figure 3).