#28 What I Learned Today

Biochemistry

Today in Biochemistry, we began to look into lipid metabolism. Lipid metabolism is centrally based around the molecule acetyl-CoA, which can be converted to fatty acids, cholesterol, HMG-CoA, through this pathway.

You see two types of fatty acids: saturated and unsaturated. Saturated fatty acids are long alkyl chains of monocarboxylic acids with an even-number of carbons. Unsaturated fatty acids differ in that they contain C=C double bonds. This results in different melting points; saturated fatty acids are straight chains, meaning they have more surface area and are less likely to be able to flow. Unsaturated fatty acids have kinks in the molecule, and thus have less surface area and more ability to flow. As unsaturation increases, melting point decreases, and as chain length increases, melting point increases.

Two important polyunsaturated fatty acids are linoleic acid and linolenic acid. They are commonly known as omega-3 and omega-6 fatty acids, respectively.

Fatty acids are formed by a number of reactions. The first one to note is the De Novo synthesis. It uses an enzyme with two binding sites: one is an acyl carrier protein, which uses pantothenic acid (found in coenzyme A) to attach fatty acids, and the other is a cysteine residue. The ACP begins by binding malonyl-Coa (an acetyl-CoA with a carboxyl group). The cysteine residue binds an acetyl-CoA. The acetyl is then removed from the cysteine and hooks onto the end of the malonyl-CoA, through the enzyme ketoacyl synthase. This reaction removes the carboxyl group in the form of carbon dioxide. At this point, the cysteine is empty, and the pantothenic acid have a four-carbon chain. The ketone group on the pantothenic acid is then reduced, which uses one NADPH (this is why the hexose monophosphate shunt is so important!). The enzyme hydratase then produces water by removing an OH group and another hydrogen, forming a trans C=C (unsautrated!). The four-carbon molecule is then saturated through the use of another NADPH in a reduction reaction. Extra carbon is then added by transferring the four-carbon molecule to a cysteine residue, addding another malonyl-CoA to the pantothenic acid, and finally transferring the four-carbon molecule onto the malonyl-CoA, releasing another carbon dioxide.