Final answer:
The fatty acid synthesis/degradation substrate with a harmful natural isomer is thalidomide, with the S isomer being the potent and teratogenic drug that led to critical drug approval regulations.
Step-by-step explanation:
The substrate in fatty acid synthesis/degradation with a naturally-occurring isomer that is a potent drug is thalidomide. The isomer referred to in this case is the 'S' isomer of thalidomide, which is a teratogen that caused birth defects when it was used by pregnant women, especially during the 1950s and early 1960s. The 'R' isomer of thalidomide, does not have these teratogenic effects but the R and S isomers can interconvert, which led to the presence of the harmful S isomer even when the pure R isomer was used. This tragedy highlighted the importance of stereochemistry in drug design and the rigorous testing of all isomers of chiral drugs before approval.
Lovastatin belongs to a class of medications known as statins, which are widely prescribed to reduce the production of cholesterol in the liver. It works by inhibiting the enzyme HMG-CoA reductase, which is involved in the synthesis of mevalonic acid. By blocking this enzyme, lovastatin effectively lowers cholesterol levels in the bloodstream, thereby reducing the risk of cardiovascular diseases.