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 Δ⁶-desaturase and all the C₂₀ PUFAs by a Δ⁵-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 (α-type desaturase such as the Δ⁵- and Δ⁶-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.