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Amino acid metabolism

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Amino acid metabolism

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Amino acid metabolism

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Only (L/D) -forms of amino acids are used by cells. 

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Content Reviewers:

Rishi Desai, MD, MPH

Amino acids are the building blocks of proteins.

And just like how you can make lots of words with a finite alphabet, it’s possible to make lots of proteins with just 20 amino acids!

Each amino acid has nitrogen-containing amine group, and a carboxylic acid - hence the name amino acid!

Each amino acid also has a unique side chain that’s kind of like the amino acid’s fingerprint.

10 of the 20 amino acids are essential, meaning that you obtain them from dietary sources rich in protein, such as meats or tofu.

The other 10 amino acids are non-essential, which means that they can be made in our body, so you don’t have to get them from your diet.

So let’s say you had a nice big bowl of lentils rich in protein.

Protein would get broken down into amino acids, and those amino acids would make their way into various cells to serve as building blocks in protein synthesis.

And the cell has to try to make use of these amino acids, because ammonia which is the nitrogen-containing amine group in amino acids, can become toxic to the cell if it gets freed up and starts to build up in the cell.

Ultimately, to get rid of it, ammonia must first be removed from the amino acid and then sent to the liver where it can get metabolized into a less toxic molecule called urea.

To do that, a group of enzymes called transaminases or aminotransferases transfer that nitrogen containing amino group from amino acids to ketoacids, like alpha-ketoglutarate.

These reactions are called transamination reactions and they’re reversible reactions, meaning that the reaction can go in either direction using the same enzyme.

And generally speaking, transamination reactions requires pyridoxine, or vitamin B6, as a cofactor to help move things along.

So let’s take an example of a transamination reaction with the amino acid alanine in a muscle cell.

First, the enzyme alanine transaminase, or ALT, switches the amino group on alanine with the oxygen group on alpha-ketoglutarate, resulting in a ketoacid called pyruvate, which now has the oxygen group, and the amino acid glutamate, which now has the amino group.

Now pyruvate has two options. First, it can be converted into acetyl-CoA by pyruvate dehydrogenase in the muscle cell, and then the acetyl-CoA can then enter the citric acid cycle.

Second, pyruvate can be converted to lactate by lactate dehydrogenase, and lactate can then travel to the liver.

Because this is a reversible reaction, lactate can be reconverted back to pyruvate in the liver by lactate dehydrogenase as well.

Within the liver, pyruvate can enter gluconeogenesis, and help form a new glucose molecule.