Fatty acid unsaturation in the red alga Porphyridium cruentum. Is the methylene interrupted nature of polyunsaturated fatty acids an intrinsic property of the desaturases?

Polyunsaturated fatty acids (PUFAs) exogenously supplied to the red microalga Porphyridium cruentum were incorporated into cellular lipids. All the C18 PUFAs studied were desaturated by a delta 6-desaturase and all the C20 PUFAs by a delta 5-desaturase. The latter enzyme desaturated even 20:2(11, 14) to 20:3(5, 11, 14) and 20:3(11, 14, 17) to 20:4(5, 11, 14, 17). We infer the existence of several fatty acid desaturases, with different chain length specificities. Furthermore, the introduction of double bonds in a methylene interrupted pattern, at least for alpha-type desaturase such as the delta 5- and delta 6-desaturases, is not an intrinsic property of the enzyme but a consequence of the arrangement of the pre-existing double bonds in the fatty acid substrate.