Specific for desaturation of the 5 position in C16 and C18 fatty acids. The histidine box domains may contain the active site and/or be involved in metal ion binding. Mutants show a significant decrease in the amount of dienoic acids with a concomitant increase in their monoenoic acid precursors, and have an increased tolerance to heat shock. Belongs to the fatty acid desaturase type 1 family.