Read for comprehension. You are training your reading skills. This is biochemistry a full two steps beyond MCAT level.

Many biological compounds are formed from isopentenyl pyrophosphate, a fundamental 5 carbon building block. Terpenes such as menthol (10 carbons), vitamin A (20 carbons), and β-carotene (40 carbons) are isoprenoid compounds, biosynthesized from the isoprene unit (5 carbons) isopentenyl pyrophosphate. This pathway also leads to steroids by way of the isoprenoic compound squalene (30 carbons).

Isopentenyl pyrophosphate is synthesized from acetyl coenzyme A . The sequence of steps is similar to fatty acid synthesis, involving addition of CO2 to Acetyl coA to form malonyl coenzyme A, which activates the α carbon to nucleophilically attach to the carbonyl carbon of another acetyl coA molecule (CO2 is lost in this stage). The result is acetoacetyl coenzyme A, a four carbon molecule. Unlike fatty acid biosynthesis, which proceeds through reduction, dehydration, then hydrogenation from this point to produce an aliphatic sequence, the synthesis of isopentenyl pyrophosphate (and ultimately terpenes and steroids), proceeds with the second carbonyl carbon of acetoacetyl coenzyme A attaching to the α carbon of another molecule of acetyl coenzyme A, creating a product (HMG CoA), which is then reduced to mevalonic acid (six carbons) by a reductase. This is the committed step in cholesterol synthesis. The reductase is an important control site. Decarboxylization leads to isopentenyl pyrophosphate (five carbons), which is the fundamental building block of terpenes. Isopentenyl pyrophosphate then combines with a structural isomer with which it interconverts, dimethylallyl pyrophosphate, leading to terpene elongation.

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