Amino acid metabolism

Definitions & Key takeaways

Amino acid metabolism refers to the biochemical pathways that produce, break down, and use amino acids. The body uses amino acids to make proteins, enzymes, hormones, and other important molecules. Protein synthesis and degradation are both essential for maintaining homeostasis in the body.

Amino acid oxidation is a process that helps the body release energy from protein molecules. This process occurs in the liver and muscles and results in the production of ketone bodies, which can be used by cells to generate energy.

Amino acids can also be used to make glucose, which is an important source of energy for the body. The body can also use amino acids to make lipids (fats) and cholesterol.

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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.
9 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 11 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 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. Glucose can then enter the circulation, and go back to the muscle cell, where it can be broken down in glycolysis to make ATP and our original friend, pyruvate.
This nifty little cycle is called the Cori cycle. Now glutamate is a unique amino acid, and it’s the only amino acid that doesn’t have to transfer its nitrogen-containing amine group to another molecule.
That’s because glutamate undergoes oxidative deamination, a process that removes hydrogens and an amino group. So specifically, what happens, is that glutamate travels to the liver mitochondria.
This process takes place in the mitochondria because free ammonia in the cytoplasm can damage the cell. So within the relatively safe environment of the mitochondria, the enzyme glutamate dehydrogenase removes glutamate’s amino group and adds an oxygen group from water - forming alpha ketoglutarate.
At the same time, glutamate’s hydrogens get transferred to nicotinamide adenine dinucleotide, or NAD+, forming NADH. The free ammonia in the mitochondria, can then be converted into the less toxic molecule urea in a process called the urea cycle.
And remember that all of these reactions are reversible. So when the body needs amino acids, these reactions can go in full reverse using the same enzymes to synthesize our original amino acid - alanine.
Now in addition to the nitrogen-containing amine group, the cell still has to metabolize the rest fo the amino acid. Some amino acids such as alanine and glycine, serve as substrates to make new glucose, we call those glucogenic amino acids.
Other amino acids, such as leucine and lysine form alternative energy fuels called ketone bodies, and we call those ketogenic.
And other amino acids, such as valine and aspartate can feed directly into the citric acid cycle, and lead to the formation of energy as ATP.
Some amino acids such as phenylalanine can do it all, being glucogenic, ketogenic and feeding into the citric acid cycle as well.
Alright, a quick recap. Amino acids have a nitrogen-containing amine group that has to be safely metabolized to avoid toxicity.
To do that, cells use transamination reactions with a ketoacid such as alpha ketoglutarate to generate intermediates like pyruvate and glutamate.
Glutamate can then be oxidatively deaminated in the liver mitochondria to get rid of ammonia in the urea cycle.