The present invention relates to proteins involved in fatty acid synthesis, such as fatty acid synthases (FAS) variants, comprising one or more polypeptide chains, wherein said polypeptide chain(s) comprise one or more subunits comprising a malonyl/palmitoyl transferase domain (MPT domain), acetyl transferase domain (AT domain), and ketoacyl synthase domain (KS domain), and at least one amino acid substitution in the MPT domain at a position corresponding to R130, in the AT domain at a position corresponding to I306, and/or in the KS domain, preferably in the acyl binding channel and/or at KS domain binding site to ACP, to modulate affinities of acyl intermediates, and optionally further amino acid substitution(s). The present invention relates to the respective polypeptide domains. The present invention further relates to nucleic acid molecules encoding the proteins (or the polypeptide domains) and to host cells containing said nucleic acid molecules. The present invention further relates to a method for the production of short fatty acids, CoA esters of short fatty acids, ethyl esters of short fatty acids, esters of short fatty acids with other metabolites, and/or enzyme bound short fatty acids (C6 to C12), comprising the expression of said nucleic acid molecules, preferably in said host cells. The present invention further relates to a method for the production of biofuels, flavoring compounds and/or fine chemicals, comprising the expression of said nucleic acid molecules, preferably in said host cells. The present invention also relates to the use of the proteins, nucleic acids molecule or host cells for the bulk production of short fatty acids (C6 to C12), the specific production of C6 fatty acids and/or C8 fatty acids, the bulk production of CoA esters of short fatty acids (C6 to C12), the specific production of C6-CoA esters and/or C8-CoA esters, the bulk production of ethyl esters of short fatty acids (C6 to C12), the specific production of C6 fatty acid ethyl esters and/or C8 fatty acid ethyl esters, the bulk production of esters of short fatty acids (C6 to C12) with other metabolites, the specific production of C6 fatty acid esters with other metabolites and/or C8 fatty acid esters with other metabolites, the bulk production of enzyme bound short fatty acids (C6 to C12), the specific production of enzyme bound C6 fatty acids and/or enzyme bound C8 fatty acids, the production of biofuels, fine chemicals and/or flavoring substances.