Fatty acids and other polyketides are made from acetyl CoA

we will show how nature can take a very simple molecule— acetyl CoA—and build it up into an amazing variety of structures. There are two main path ways from acetyl CoA through malonyl CoA and mevalonic acid and each gives rise to two important series of natural products. Malonyl CoA leads to fatty acids and polyketides while mevalonic acid gives terpenes and steroids. We start with the simplest, the fatty acids. The list below shows just a few of the fatty acids that exist: all are present in a typical diet and you’ll find many referred to on the labels of processed foods.

Fatty acids have some important features which you should note: • They have straight chains with no branching. • They have even numbers of carbon atoms. • They may be saturated with no double bonds in the chain or they may have one or more C=C double bonds in the chain, in which case they are usually cis (Z) alkenes. If there is more than one C=C double bond, they are not conjugated (either with the CO2H group or with each other)—there is normally one saturated carbon atom between them. Palmitic acid (C16 saturated) is the most common fatty acid in living things. Oleic acid (C18 mono-unsaturated) is the major fatty acid in olive oil. Arachidonic acid (C20 tetra-unsaturated)

is a rare fatty acid, which is the precursor of the very important biological messengers the prostaglandins, thromboxanes, and leukotrienes. The prevalence of fatty acids with even numbers of carbon atoms suggests a two-carbon building block, the most obvious being acetate. If labelled acetate is fed to plants, the fatty acids emerge with labels on alternate carbons like this.

The green blob might represent deuterium (as a CD3 group) and the black blob 13C. In fact, the reactions are more complex than this suggests as CO₂ is also needed as well as CoA and it turns out that only the fi rst two-carbon unit is put in as acetyl CoA. The remainder are added as malonyl CoA. If labelled malonyl CoA is fed, the starter unit, as it is called, is not labelled.

The fi rst stage in fatty acid biosynthesis is a condensation between acetyl CoA (the starter unit) and malonyl CoA with the loss of CO2. This reaction could be drawn like this, with CO2 being lost as the new C–C bond is formed. When chemists use malonates, we like to make the stable enol using both carbonyl groups, condense, and only afterwards release CO₂, nature does this in making acetoacetyl CoA during alkaloid bio synthesis, but here things work differently.

The next step is reduction of the ketone group. This NADPH reaction is typically stereo- and chemoselective, although the stereochemistry is rather wasted here as the next step is a de hydration, typical of what is now an aldol product, and occurring by an enzyme-catalysed E1cB mechanism. The elimination is known to be a cis removal of H and OH, and the double bond is exclusively trans (E). Finally in this cycle, the double bond is reduced using another molecule of NADPH to give the saturated side chain.

Now the whole cycle can start again using this newly made C4 fatty acid as the starter unit and building a C6 fatty acid and so on. Each time the cycle turns, two carbon atoms are added to the acyl end of the growing chain.

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مواضيع ذات صلة

The future of organic chemistry

The synthesis of oseltamivir

The synthesis of indinavir

Chemistry vs viruses

Terpenes are volatile constituents of plants

Aromatic polyketides

Benzyl isoquinoline alkaloids are made from tyrosine

Pyrrolidine alkaloids are made from the amino acid ornithine

Alkaloids are made by amino acid metabolism

The shikimic acid pathway

Phosphoenolpyruvate

Nature’s enolate equivalents: lysine enamines and coenzyme A