Beta-carotene ketolase/oxygenase (CrtW, also known as CrtO), the carotenoid astaxanthin biosynthetic enzyme, initially catalyzes the addition of two keto groups to carbons C4 and C4' of beta-carotene. Carotenoids are important natural pigments produced by many microorganisms and plants. Astaxanthin is reported to be an antioxidant, an anti-cancer agent, and an immune system stimulant. A number of bacteria and green algae can convert beta-carotene into astaxanthin by using several ketocarotenoids as intermediates and CrtW and a beta-carotene hydroxylase (CrtZ). CrtW initially converts beta-carotene to canthaxanthin via echinenone, and CrtZ initially mediates the conversion of beta-carotene to zeaxanthin via beta-cryptoxanthin. After a few more intermediates are formed, CrtW and CrtZ act in combination to produce astaxanthin. Sequences of this domain family appear to be structurally related to membrane fatty acid desaturases and alkane hydroxylases. They all share in common extensive hydrophobic regions that are capable of spanning the membrane bilayer at least twice. Comparison of these sequences also reveals three regions of conserved histidine cluster motifs that contain eight histidine residues: HXXXH, HXXHH, and HXXHH. These histidine residues are reported to be catalytically essential and proposed to be the ligands for the iron atoms contained within homologs, stearoyl CoA desaturase and alkane hydroxylase.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126  30 GIFIACFILFLWAISLILLLSIDTSIIHKSLLGIAMLWQTFLYTGLFITAHDAMHGVVYPKNPRINNFIGKLTLILYGLL 109
Cdd:cd03513   1 GLTLAGLIIAAWLASHVHALFFPRGSISLLLIAAVVLLQTFLHTGLFIIAHDAMHGSLAPGNPRLNRAIGRLCLFLYAGF 80

                        90       100       110       120       130       140       150       160
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126 110 PYKDLLKKHWLHHGHPGTDLDPDYYNGHPQNFFLWYLHFMKSYWRWTQIFGLVMIFHGLKNLVHIPENNLIIFWMIPSIL 189
Cdd:cd03513  81 SYDRLLRKHHLHHRHPGTAKDPDFHKGNPSGFLPWYLHFMSNYFGWRQLAILAAVWLLLLGLGSAPLANLLLFWALPLIL 160

                       170       180       190       200       210       220
                ....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 54304126 190 SSVQLFYFGTFLPHKKLEGGYTNPHCARSIPLPLFWSFVTCYHFGYHKEHHEYPQLPWWKLPEAH 254
Cdd:cd03513 161 SSLQLFYFGTWLPHRPGRGGFADRHRARSSRLSPVLSFLTCYHFGYHHEHHLSPSTPWWRLPELR 225

Beta-carotene ketolase/oxygenase (CrtW, also known as CrtO), the carotenoid astaxanthin biosynthetic enzyme, initially catalyzes the addition of two keto groups to carbons C4 and C4' of beta-carotene. Carotenoids are important natural pigments produced by many microorganisms and plants. Astaxanthin is reported to be an antioxidant, an anti-cancer agent, and an immune system stimulant. A number of bacteria and green algae can convert beta-carotene into astaxanthin by using several ketocarotenoids as intermediates and CrtW and a beta-carotene hydroxylase (CrtZ). CrtW initially converts beta-carotene to canthaxanthin via echinenone, and CrtZ initially mediates the conversion of beta-carotene to zeaxanthin via beta-cryptoxanthin. After a few more intermediates are formed, CrtW and CrtZ act in combination to produce astaxanthin. Sequences of this domain family appear to be structurally related to membrane fatty acid desaturases and alkane hydroxylases. They all share in common extensive hydrophobic regions that are capable of spanning the membrane bilayer at least twice. Comparison of these sequences also reveals three regions of conserved histidine cluster motifs that contain eight histidine residues: HXXXH, HXXHH, and HXXHH. These histidine residues are reported to be catalytically essential and proposed to be the ligands for the iron atoms contained within homologs, stearoyl CoA desaturase and alkane hydroxylase.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126  30 GIFIACFILFLWAISLILLLSIDTSIIHKSLLGIAMLWQTFLYTGLFITAHDAMHGVVYPKNPRINNFIGKLTLILYGLL 109
Cdd:cd03513   1 GLTLAGLIIAAWLASHVHALFFPRGSISLLLIAAVVLLQTFLHTGLFIIAHDAMHGSLAPGNPRLNRAIGRLCLFLYAGF 80

                        90       100       110       120       130       140       150       160
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126 110 PYKDLLKKHWLHHGHPGTDLDPDYYNGHPQNFFLWYLHFMKSYWRWTQIFGLVMIFHGLKNLVHIPENNLIIFWMIPSIL 189
Cdd:cd03513  81 SYDRLLRKHHLHHRHPGTAKDPDFHKGNPSGFLPWYLHFMSNYFGWRQLAILAAVWLLLLGLGSAPLANLLLFWALPLIL 160

                       170       180       190       200       210       220
                ....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 54304126 190 SSVQLFYFGTFLPHKKLEGGYTNPHCARSIPLPLFWSFVTCYHFGYHKEHHEYPQLPWWKLPEAH 254
Cdd:cd03513 161 SSLQLFYFGTWLPHRPGRGGFADRHRARSSRLSPVLSFLTCYHFGYHHEHHLSPSTPWWRLPELR 225

Fatty acid desaturase; Fatty acid desaturases are enzymes that catalyze the insertion of a double bond at the delta position of fatty acids. There seem to be two distinct families of fatty acid desaturases which do not seem to be evolutionary related: Family 1 composed of Stearoyl-CoA desaturases (SCD) and Family 2 composed of Bacterial fatty acid desaturases, Plant stearoyl-acyl-carrier-protein desaturase and Cyanobacterial DesA. Members of this entry are ER integral membrane proteins that share the same mushroom-shaped fold consisting of four transmembrane helices (TM1-TM4) which anchor them to the membrane, capped by a cytosolic domain containing a unique 9-10 histidine- coordinating di metal (di-iron) catalytic centre. The structure of mouse stearoyl-CoA desaturase (SDC) revealed that TM2 and TM4 are longer than TM1 and TM3 and protrude into the cytosolic domain, providing three of the nine histidine residues that coordinate the two metal ions, while the other histidine residues are provided by the soluble domain in this enzyme.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126    69 TFLYTGLFITAHDAMHGVVYPK---NPRINNFIGKLTLILYGLlPYKDLLKKHWLHHGHP-GTDLDPDY------YNGHP 138
Cdd:pfam00487  13 LFLLGITGSLAHEASHGALFKKrrlNRWLNDLLGRLAGLPLGI-SYSAWRIAHLVHHRYTnGPDKDPDTaplasrFRGLL 91

                          90       100       110       120       130       140       150       160
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126   139 QNFFLWYLH-----------------------FMKSYWRWTQIFGLVMIFH----GLKNLVHIPENNLIIFWMIPSILSS 191
Cdd:pfam00487  92 RYLLRWLLGllvlawllalvlplwlrrlarrkRPIKSRRRRWRLIAWLLLLaawlGLWLGFLGLGGLLLLLWLLPLLVFG 171

                         170       180       190       200       210       220
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 54304126   192 VQLFYFGTFLPHKKLEGGYTNPHCARSIPLPLFWSFVTCYHFGYHKEHHEYPQLPWWKLPEAHKI 256
Cdd:pfam00487 172 FLLALIFNYLEHYGGDWGERPVETTRSIRSPNWWLNLLTGNLNYHIEHHLFPGVPWYRLPKLHRR 236

Beta-carotene ketolase/oxygenase (CrtW, also known as CrtO), the carotenoid astaxanthin biosynthetic enzyme, initially catalyzes the addition of two keto groups to carbons C4 and C4' of beta-carotene. Carotenoids are important natural pigments produced by many microorganisms and plants. Astaxanthin is reported to be an antioxidant, an anti-cancer agent, and an immune system stimulant. A number of bacteria and green algae can convert beta-carotene into astaxanthin by using several ketocarotenoids as intermediates and CrtW and a beta-carotene hydroxylase (CrtZ). CrtW initially converts beta-carotene to canthaxanthin via echinenone, and CrtZ initially mediates the conversion of beta-carotene to zeaxanthin via beta-cryptoxanthin. After a few more intermediates are formed, CrtW and CrtZ act in combination to produce astaxanthin. Sequences of this domain family appear to be structurally related to membrane fatty acid desaturases and alkane hydroxylases. They all share in common extensive hydrophobic regions that are capable of spanning the membrane bilayer at least twice. Comparison of these sequences also reveals three regions of conserved histidine cluster motifs that contain eight histidine residues: HXXXH, HXXHH, and HXXHH. These histidine residues are reported to be catalytically essential and proposed to be the ligands for the iron atoms contained within homologs, stearoyl CoA desaturase and alkane hydroxylase.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126  30 GIFIACFILFLWAISLILLLSIDTSIIHKSLLGIAMLWQTFLYTGLFITAHDAMHGVVYPKNPRINNFIGKLTLILYGLL 109
Cdd:cd03513   1 GLTLAGLIIAAWLASHVHALFFPRGSISLLLIAAVVLLQTFLHTGLFIIAHDAMHGSLAPGNPRLNRAIGRLCLFLYAGF 80

                        90       100       110       120       130       140       150       160
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126 110 PYKDLLKKHWLHHGHPGTDLDPDYYNGHPQNFFLWYLHFMKSYWRWTQIFGLVMIFHGLKNLVHIPENNLIIFWMIPSIL 189
Cdd:cd03513  81 SYDRLLRKHHLHHRHPGTAKDPDFHKGNPSGFLPWYLHFMSNYFGWRQLAILAAVWLLLLGLGSAPLANLLLFWALPLIL 160

                       170       180       190       200       210       220
                ....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 54304126 190 SSVQLFYFGTFLPHKKLEGGYTNPHCARSIPLPLFWSFVTCYHFGYHKEHHEYPQLPWWKLPEAH 254
Cdd:cd03513 161 SSLQLFYFGTWLPHRPGRGGFADRHRARSSRLSPVLSFLTCYHFGYHHEHHLSPSTPWWRLPELR 225

Fatty acid desaturase; Fatty acid desaturases are enzymes that catalyze the insertion of a double bond at the delta position of fatty acids. There seem to be two distinct families of fatty acid desaturases which do not seem to be evolutionary related: Family 1 composed of Stearoyl-CoA desaturases (SCD) and Family 2 composed of Bacterial fatty acid desaturases, Plant stearoyl-acyl-carrier-protein desaturase and Cyanobacterial DesA. Members of this entry are ER integral membrane proteins that share the same mushroom-shaped fold consisting of four transmembrane helices (TM1-TM4) which anchor them to the membrane, capped by a cytosolic domain containing a unique 9-10 histidine- coordinating di metal (di-iron) catalytic centre. The structure of mouse stearoyl-CoA desaturase (SDC) revealed that TM2 and TM4 are longer than TM1 and TM3 and protrude into the cytosolic domain, providing three of the nine histidine residues that coordinate the two metal ions, while the other histidine residues are provided by the soluble domain in this enzyme.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126    69 TFLYTGLFITAHDAMHGVVYPK---NPRINNFIGKLTLILYGLlPYKDLLKKHWLHHGHP-GTDLDPDY------YNGHP 138
Cdd:pfam00487  13 LFLLGITGSLAHEASHGALFKKrrlNRWLNDLLGRLAGLPLGI-SYSAWRIAHLVHHRYTnGPDKDPDTaplasrFRGLL 91

                          90       100       110       120       130       140       150       160
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126   139 QNFFLWYLH-----------------------FMKSYWRWTQIFGLVMIFH----GLKNLVHIPENNLIIFWMIPSILSS 191
Cdd:pfam00487  92 RYLLRWLLGllvlawllalvlplwlrrlarrkRPIKSRRRRWRLIAWLLLLaawlGLWLGFLGLGGLLLLLWLLPLLVFG 171

                         170       180       190       200       210       220
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 54304126   192 VQLFYFGTFLPHKKLEGGYTNPHCARSIPLPLFWSFVTCYHFGYHKEHHEYPQLPWWKLPEAHKI 256
Cdd:pfam00487 172 FLLALIFNYLEHYGGDWGERPVETTRSIRSPNWWLNLLTGNLNYHIEHHLFPGVPWYRLPKLHRR 236

sphingolipid delta-4 desaturase

                         10        20        30        40        50        60        70        80
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126   34 ACFILFLWAISLILLLSIDTSIIHKSLLGIAMLWQTFLYTGLFITAHDAMHGVVYpKNPRINNFIGKLTLILYGLLPYKD 113
Cdd:PLN02579  47 AFPKIAAVVLLQLCTATLLHDAGWPKILLVAYFFGGFLNHNLFLAIHELSHNLAF-KTPVYNRWLGIFANLPIGIPMSVT 125

                         90       100       110       120       130       140       150       160
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126  114 LLKKHWLHH---GHPGTDLD-PDYYNGHpqnffLWYLHFMKSYWRWTQIFglvmiFHGLKNLVHIPEN-------NLII- 181
Cdd:PLN02579 126 FQKYHLEHHrfqGVDGIDMDiPSQGEAR-----LVRNTLSKIVWVFLQLF-----FYALRPLFVNPKPpglwefiNLLTq 195

                        170       180       190       200       210       220       230       240
                 ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126  182 ---------FWMIPSILSSVqlfyFGTFLphkkleGGYTNPHCARSI-----------------PLPLFwsfvtCYHFGY 235
Cdd:PLN02579 196 iafdaalvyFAGWKSLAYLI----LSTFL------GGGLHPMAGHFIsehyvfnpgqetysyygPLNLL-----TWNVGY 260

                        250       260
                 ....*....|....*....|..
gi 54304126  236 HKEHHEYPQLPWWKLPEAHKIS 257
Cdd:PLN02579 261 HNEHHDFPRIPGSKLHKVKEIA 282

The membrane fatty acid desaturase (Membrane_FADS)-like CD includes membrane FADSs, alkane hydroxylases, beta carotene ketolases (CrtW-like), hydroxylases (CrtR-like), and other related proteins. They are present in all groups of organisms with the exception of archaea. Membrane FADSs are non-heme, iron-containing, oxygen-dependent enzymes involved in regioselective introduction of double bonds in fatty acyl aliphatic chains. They play an important role in the maintenance of the proper structure and functioning of biological membranes. Alkane hydroxylases are bacterial, integral-membrane di-iron enzymes that share a requirement for iron and oxygen for activity similar to that of membrane FADSs, and are involved in the initial oxidation of inactivated alkanes. Beta-carotene ketolase and beta-carotene hydroxylase are carotenoid biosynthetic enzymes for astaxanthin and zeaxanthin, respectively. This superfamily domain has extensive hydrophobic regions that would be capable of spanning the membrane bilayer at least twice. Comparison of these sequences also reveals three regions of conserved histidine cluster motifs that contain eight histidine residues: HXXX(X)H, HXX(X)HH, and HXXHH (an additional conserved histidine residue is seen between clusters 2 and 3). Spectroscopic and genetic evidence point to a nitrogen-rich coordination environment located in the cytoplasm with as many as eight histidines coordinating the two iron ions and a carboxylate residue bridging the two metals in the Pseudomonas oleovorans alkane hydroxylase (AlkB). In addition, the eight histidine residues are reported to be catalytically essential and proposed to be the ligands for the iron atoms contained within the rat stearoyl CoA delta-9 desaturase.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126  61 LGIAMLWQTFLYTGLFITAHDAMHGVVYpKNPRINNFIGkLTLILYGLLPYKDLLKKHWLHHGHPGTDL-DPDYYNGHPQ 139
Cdd:cd01060   1 LLLALLLGLLGGLGLTVLAHELGHRSFF-RSRWLNRLLG-ALLGLALGGSYGWWRRSHRRHHRYTNTPGkDPDSAVNYLE 78

                        90
                ....*....|....*..
gi 54304126 140 NFFLWYLHFMKSYWRWT 156
Cdd:cd01060  79 HYGGDRPFDTDGEWLRT 95

Beta-carotene hydroxylase (CrtR), the carotenoid zeaxanthin biosynthetic enzyme catalyzes the addition of hydroxyl groups to the beta-ionone rings of beta-carotene to form zeaxanthin and is found in bacteria and red algae. Carotenoids are important natural pigments; zeaxanthin and lutein are the only dietary carotenoids that accumulate in the macular region of the retina and lens. It is proposed that these carotenoids protect ocular tissues against photooxidative damage. CrtR does not show overall amino acid sequence similarity to the beta-carotene hydroxylases similar to CrtZ, an astaxanthin biosynthetic beta-carotene hydroxylase. However, CrtR does show sequence similarity to the green alga, Haematococcus pluvialis, beta-carotene ketolase (CrtW), which converts beta-carotene to canthaxanthin. Sequences of the CrtR_beta-carotene-hydroxylase domain family, as well as, the CrtW_beta-carotene-ketolase domain family appear to be structurally related to membrane fatty acid desaturases and alkane hydroxylases. They all share in common extensive hydrophobic regions that would be capable of spanning the membrane bilayer at least twice. Comparison of these sequences also reveals three regions of conserved histidine cluster motifs that contain eight histidine residues: HXXXH, HXXHH, and HXXHH. These histidine residues are reported to be catalytically essential and proposed to be the ligands for the iron atoms contained within homologs, stearoyl CoA desaturase and alkane hydroxylase.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126  40 LWAISL-ILLLSIDTSIIHKSLLGIAMLWQTFLYTGLFITAHDAMHGVVYPkNPRINNFIGKLTLILYGLlPYKDLLKKH 118
Cdd:cd03514   2 LFLISMaLVWLSTWGYVISYLPLWVCFILNTLSLHLAGTVIHDASHKAASR-NRWINELIGHVSAFFLGF-PFPVFRRVH 79

                        90       100       110       120       130       140       150       160
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 54304126 119 WLHHGHP-GTDLDPDYYnghpqnfflwylhFMKSYWRWTQIFGLVMIFHGLKNLvhipeNNLIIFWMIPSILSSVQLFYF 197
Cdd:cd03514  80 MQHHAHTnDPEKDPDHF-------------LLEWLVARSLFITLLVIAILFGFL-----WELLNLWFLPALIVGTYLALF 141

                       170       180       190       200       210
                ....*....|....*....|....*....|....*....|....*....|....*..
gi 54304126 198 GTFLPHKKLE-GGYTNPHCARSIPLPLFWSFVTCYHFgyhkEHHEYPQLPWWKLPEA 253
Cdd:cd03514 142 FDWLPHHPFEeTQRWDNSRVYPSKLLNPLIMGQNYHL----VHHLWPSIPWYRYPEA 194

The Delta6 Fatty Acid Desaturase (Delta6-FADS)-like CD includes the integral-membrane enzymes: delta-4, delta-5, delta-6, delta-8, delta-8-sphingolipid, and delta-11 desaturases found in vertebrates, higher plants, fungi, and bacteria. These desaturases are required for the synthesis of highly unsaturated fatty acids (HUFAs), which are mainly esterified into phospholipids and contribute to maintaining membrane fluidity. While HUFAs may be required for cold tolerance in bacteria, plants and fish, the primary role of HUFAs in mammals is cell signaling. These enzymes are described as front-end desaturases because they introduce a double bond between the pre-exiting double bond and the carboxyl (front) end of the fatty acid. Various substrates are involved, with both acyl-coenzyme A (CoA) and acyl-lipid desaturases present in this CD. Acyl-lipid desaturases are localized in the membranes of cyanobacterial thylakoid, plant endoplasmic reticulum (ER), and plastid; and acyl-CoA desaturases are present in ER membrane. ER-bound plant acyl-lipid desaturases and acyl-CoA desaturases require cytochrome b5 as an electron donor. Most of the eukaryotic desaturase domains have an adjacent N-terminal cytochrome b5-like domain. This domain family has extensive hydrophobic regions that would be capable of spanning the membrane bilayer at least twice. Comparison of sequences also reveals the existence of three regions of conserved histidine cluster motifs that contain the residues: HXXXH, HXX(X)HH, and Q/HXXHH. These histidine residues are reported to be catalytically essential and proposed to be the ligands for the iron atoms contained within the homolog, stearoyl CoA desaturase.

                        10        20        30        40        50        60        70
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 54304126  79 AHDAMHGVVYpKNPRINNFIGKLTLILYGlLPYKDLLKKHWLHHGHPGT-DLDPDyYNGHPQNFFLWYLHFMKSYWRW 155
Cdd:cd03506  18 AHDAGHGQVF-KNRWLNKLLGLTVGNLLG-ASAGWWKNKHNVHHAYTNIlGHDPD-IDTLPLLARSEPAFGKDQKKRF 92