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Nomenclature for PK and NRP building blocks. R1 in alpha position is defined by the substrate choice of the AT- or A-domain. The α-position of PK substrates can be further modified co-synthetically by C-methyltransferases. The stereo configuration of the α-carbon of NRP substrates is defined by the presence of an E domain. The β-position of PK substrates can be further reduced. For NRP substrates, only the nitrogen atom, also referred to as β-position, can be methylated.
