>gnl|CDD|173831 cd00683, Trans_IPPS_HH, Trans-Isoprenyl Diphosphate Synthases, head-to-head. These trans-Isoprenyl Diphosphate Synthases (Trans_IPPS) catalyze a head-to-head (HH) (1'-1) condensation reaction. This CD includes squalene and phytoene synthases which catalyze the 1'-1 condensation of two 15-carbon (farnesyl) and 20-carbon (geranylgeranyl) isoprenyl diphosphates, respectively. The catalytic site consists of a large central cavity formed by mostly antiparallel alpha helices with two aspartate-rich regions (DXXXD) located on opposite walls. These residues mediate binding of prenyl phosphates. A two-step reaction has been proposed for squalene synthase (farnesyl-diphosphate farnesyltransferase) in which, two molecules of FPP react to form a stable cyclopropylcarbinyl diphosphate intermediate, and then the intermediate undergoes heterolysis, isomerization, and reduction with NADPH to form squalene, a precursor of cholestrol. The carotenoid biosynthesis enzyme, phytoene synthase (CrtB), catalyzes the condensation reaction of two molecules of geranylgeranyl diphosphate to produce phytoene, a precursor of beta-carotene. These enzymes produce the triterpene and tetraterpene precursors for many diverse sterol and carotenoid end products and are widely distributed among eukareya, bacteria, and archaea. Length = 265

 Score = 42.6 bits (101), Expect = 1e-04
 Identities = 43/223 (19%), Positives = 83/223 (37%), Gaps = 28/223 (12%)

Query: 36  YLACLLSPPLYRISLSFLYYFHTELMR-----VRDTARNPITRDMRLQWWKDIFESSTKG 90
           YLA  L PP  R ++  LY F     R     V D A  P  +   L  ++   +++  G
Sbjct: 15  YLASRLLPPELRRAVCALYAF----CRAADDIVDDPAAPPDEKLALLDAFRAELDAAYWG 70

Query: 91  LIAESISPFSVELLSIVRQYDLPNQYFLDMIEAHFFDSYNDSIFDCKQFEHYAFRISS-- 148
                  P    L  + R+Y +P + F D++     D          + + Y + ++   
Sbjct: 71  GAPTH--PVLRALADLARRYGIPREPFRDLLAGMAMDLDKRRYETLDELDEYCYYVAGVV 128

Query: 149 -RL---IHLATMILDSERYSASL-RVIKYAGIAQFIGQLICQLPIHYHRGQLYFPLDILG 203
             +   +  A+    +   + +L   ++   I + +G+          RG++Y P + L 
Sbjct: 129 GLMLLRVFGASSDEAALERARALGLALQLTNILRDVGE-------DARRGRIYLPREELA 181

Query: 204 AVGLDRESFLSGQNSDRISLAIKIFAELGLKYLFKAREEMRYI 246
             G+  E  L+ +NS        +   L  +     RE +  +
Sbjct: 182 RFGVTLEDLLAPENSPAF---RALLRRLIARARAHYREALAGL 221