>TIGR00171 leuD 3-isopropylmalate dehydratase, small subunit; InterPro: IPR004431 3-isopropylmalate dehydratase (or isopropylmalate isomerase; 4.2.1.33 from EC) catalyses the stereo-specific isomerisation of 2-isopropylmalate and 3-isopropylmalate, via the formation of 2-isopropylmaleate. This enzyme performs the second step in the biosynthesis of leucine, and is present in most prokaryotes and many fungal species. The prokaryotic enzyme is a heterodimer composed of a large (LeuC) and small (LeuD) subunit, while the fungal form is a monomeric enzyme. Both forms of isopropylmalate are related and are part of the larger aconitase family . Aconitases are mostly monomeric proteins which share four domains in common and contain a single, labile [4Fe-4S] cluster. Three structural domains (1, 2 and 3) are tightly packed around the iron-sulphur cluster, while a fourth domain (4) forms a deep active-site cleft. The prokaryotic enzyme is encoded by two adjacent genes, leuC and leuD, corresponding to aconitase domains 1-3 and 4 respectively , . LeuC does not bind an iron-sulphur cluster. It is thought that some prokaryotic isopropylamalate dehydrogenases can also function as homoaconitase 4.2.1.36 from EC, converting cis-homoaconitate to homoisocitric acid in lysine biosynthesis . Homoaconitase has been identified in higher fungi (mitochondria) and several archaea and one thermophilic species of bacteria, Thermus thermophilus . This entry represents a region of the small subunit. The structure of the Pyrococcus horikoshii small subunit (O59393 from SWISSPROT) has recently been determined . As expected the structure of this polypeptide is similar to that of aconitase domain 4, though one alpha helix is replaced by a short loop with relatively high temperature factor values. This loop region is thought to be important for substrate recognition. Unlike other aconitase family proteins, this subunit formed a tetramer through disulphide linkages, though it is not expected to interfere with its interaction with the large subunit. These disulphide linkages would be expected to confer thermostability on the enzyme, reflecting the thermophilic lifestyle of the organism. Homoaconitase, aconitase, and 3-isopropylmalate dehydratase have similar overall structures. All are dehydratases (4.2.1 from EC) and bind a [4Fe-4S]-cluster. 3-isopropylmalate dehydratase is split into large (leuC) and small (leuD) chains in eubacteria. Several pairs of archaeal proteins resemble the leuC and leuD pair in length and sequence but even more closely resemble the respective domains of homoaconitase, and their identity is uncertain. The archaeal leuD-like proteins are not included in group.; GO: 0003861 3-isopropylmalate dehydratase activity, 0009098 leucine biosynthetic process, 0009316 3-isopropylmalate dehydratase complex.

Probab=97.84  E-value=6.2e-06  Score=62.44  Aligned_cols=119  Identities=22%  Similarity=0.402  Sum_probs=76.0

Q ss_pred             CCCCCHHHCCCCCCCCCCCCHHHHHHHCCCCHHHHHHHHHHCCCHHHHHHHHHCCCHHHHHHCCCCCCCCCCEEECCCCC
Q ss_conf             58857233164565554473788898749886563145653023577766520360122321257776577224538997
Q gi|254780451|r  673 GDKITTDHISPAGSIPLQSAAASYLRQRGVKEKDFNQFGTRRGNHEVMMRGTFSNIRICNYMLGEEGKKGGYTIHYPSKE  752 (896)
Q Consensus       673 GD~iTTDHISPAG~I~~~spAg~yL~~~Gv~~~dfNsygsRRgNhevM~RGtFaNirl~N~l~~~~~~egg~T~~~p~~~  752 (896)
T Consensus        17 ~~~~d~d~~~p~----------~~l~~--~~~~g~-------------g~~~~~~w~~~~d~~g-~~p~p~~~l~~p~~~   70 (194)
T TIGR00171        17 AANVDTDAIIPK----------QFLKR--ITRTGF-------------GKHLFYDWRFLKDDEG-KEPNPDFVLNFPPYQ   70 (194)
T ss_pred             CCCCCCHHHHHH----------HHHHH--HHHCCC-------------CCHHHHHHHHHCCCCC-CCCCCCEEECCCCCC
T ss_conf             122540010027----------88877--764022-------------0011111111001257-777763464144435


Q ss_pred             EEEHHHHHHHHHHCCCCEEEECCCCCCCCCHHHHHHHHHHHCCEEEEEECCHHHHHHHCCCCCEEEEEECCCCCCHHHH
Q ss_conf             8737999998998199889994677786730799988667617089997647898885053552322331789898881
Q gi|254780451|r  753 ELFIYDAAMRYKVDQVPLVVFAGVEYGNGSSRDWAAKGTRLLGVRSVIAESFERIHRSNLIGMGVIPFAFGKGISWKNL  831 (896)
Q Consensus       753 ~~~iydaA~~Y~~~~~plVViAG~~YG~GSSRdwAAkgp~lLGVkAVIA~SFeRIHrsNLv~mGiLPL~F~~~~~~~~L  831 (896)
T Consensus        71 --------------g~~~l~~~g~~~gcg~~~~~~pw~l~d~g~~~~~~p~~~d~~~~~~~~~~~lp~~~~~~~~~~~~  135 (194)
T TIGR00171        71 --------------GASILLAGGENFGCGSSREHAPWALDDYGFKVIIAPSFADIFYNNSFKNGLLPIRLPEDEVEELF  135 (194)
T ss_pred             --------------CCEEEEECCCCCCCCCCCCCCCCEECCCCEEEEECCCHHHHHHCCCCCCCCCCCCCCHHHHHHHH
T ss_conf             --------------61478862763456766444661000136246651311123312322345110002677888776