3-methylcrotonic acid decarboxylase (mdc) variants

A variant of a 3-methylcrotonic acid decarboxylase (MDC) showing an improved activity in converting 3-methylcrotonic acid into isobutene over the corresponding MDC from which it is derived and having an amino acid sequence as shown in SEQ ID NO:1 or an amino acid sequence having at least 55% sequence identity to SEQ ID NO:1, in which one or more amino acid residues at a position selected from the group consisting of positions 7, 17, 27, 33, 35, 45, 46, 48, 51, 140, 144, 183, 184, 185, 222, 227, 284, 285, 286, 287, 288, 289, 290, 292, 321, 322, 327, 329, 330, 331, 337, 338, 355, 365, 378, 380, 384, 387, 388, 389, 391, 392, 393, 394, 395, 419, 424, 425, 428, 429, 431, 432, 434, 436, 437, 438, 439, 443, 444, 446, 447, 448, 449, 451, 453, 457, 458, 459, 460, 462, 464, 465, 467, 468, 470 and 471 in the amino acid sequence shown in SEQ ID NO:1 or at a position corresponding to any of these positions, are substituted with another amino acid residue or deleted or wherein an insertion has been effected at one or more of these positions.