Final answer:
Bypassing the first β-oxidation step when oxidizing unsaturated fatty acids with a trans double bond between the alpha and beta carbons omits the need for FAD oxidation, which is otherwise required for saturated fatty acids. Additional isomerization steps are needed for unsaturated fatty acids to position double bonds correctly, leading to a slightly lower total ATP yield than saturated fatty acids.
Step-by-step explanation:
When oxidizing unsaturated fatty acids, bypassing the first step of β-oxidation is significant because this step, which involves oxidation by FAD, is not needed for unsaturated fatty acids with a trans double bond between the alpha (α) and beta (β) carbon atoms. Unsaturated fatty acids with cis double bonds or trans double bonds not between α and β carbon atoms require additional steps involving isomerization and sometimes reduction. These steps are necessary to position the double bonds correctly for the enzymatic machinery of β-oxidation to continue. As a result, the total ATP yield from unsaturated fatty acids, considering the entire process of their breakdown, is slightly less compared to their saturated counterparts due to the missing FADH2 that would have been produced in the first step for saturated fatty acids.