Rapid Review of Amino Acid Metabolism Cell Biology > Protein Synthesis > Protein Synthesis
AMINO ACID POOL
OVERVIEW
Amino acid pool
• Describes all free amino acids in the body
• Amino acids: body's primary source of nitrogen
AMINO ACID STRUCTURE
• Central carbon bound to: -H, -COOH, -NH3, R-group
• R-group: acidic (-), basic (+), branched, etc.
AMINO ACID POOL: INPUTS AND OUTPUTS Input
• Dietary proteins: absorbed as amino acids/peptides in small intestine
– Pass through hepatic portal vein and immediately enter liver – Liver plays central role in fates of free amino acids
• De novo amino acid synthesis: use C-skeletons & alpha-amino group of other amino acids to synthesize new ones
• Protein turnover: breakdown of cellular proteins
Output
• Oxidation: excess amino acids degraded to alpha amino group & C-skeleton
– Urea cycle: liver rids body of nitrogen waste – Energy production: C-skeletons enter glycolysis or citric acid cycle – Gluconeogenesis: occurs in liver – Storage: amino acids can't be stored --> C-skeletons stored as glycogen/fats
• Synthesis of nitrogen-containing molecules
– Amino acids – Neurotransmitters – Nucleotides (purines and pyrimidines) – Creatine, etc.
• Protein turnover: synthesis of new proteins
1 / 5 Protein turnover Cyclic synthesis and breakdown of proteins in the body
• Produces and consumes free amino acids
• Protein synthesis: depletes pool of free amino acids
• Protein breakdown: replenishes pool
ESSENTIAL AMINO ACIDS Must be obtained from diet
• "My Tall Vegan Friend Is Watering Kale Leaves" – M, T, V, F, I, W, K, L
• Glucogenic: Methionine, threonine, valine
• Glucogenic & ketogenic: Phenylalanine, isoleucine, tryptophan
Branched chain amino acids (BCAA)
• Valine, leucine, isoleucine
• Immediately shunt from liver to circulation
• Liver lacks an aminotransferase that muscle, adipose, kidney & brain have
Aminotransferases: family of enzymes involved in amino acid breakdown NONESSENTIAL AMINO ACIDS Synthesized endogenously (The rest of the amino acids) Glucogenic: alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, histidine, proline & serine
• Glucogenic and ketogenic: tyrosine
• Histidine is often considered essential because it is essential during the first 5 years of life but later becomes nonessential.
Note that there are many ways to categorize essential/nonessential amino acids CLINICAL CORRELATION Conditionally essential amino acids
• Cannot be synthesized under certain physiologic conditions
• Infants • Immunosuppressed patients/postoperative infections: treat w/ arginine & glutamine supplements
FULL TEXT
2 / 5 Here we will learn an overview of amino acid metabolism, the primary source of nitrogen in the body. We will specifically learn about the amino acid pool.
THE AMINO ACID POOL 
Definitions
• The amino acid pool describes all the free amino acids in the body.
• Amino acids are the body's primary source of nitrogen. These include intracellular free amino acids as well as those in circulation.
The structure of an amino acid
• Draw a central carbon bound to a hydrogen.
• Show that it binds an amino group on one side and a carboxyl group on the other.
• Indicate that its fourth bond is with an R-group, a functional side group.
Recall that the functional group can be acidic, basic, branched, etc.
• Acidic amino acids are negatively charged at physiologic pH, and that basic amino acids are positively charged.
Now that we've reviewed some basic characteristics of amino acids, let's learn how the content of our amino acid pool fluctuates.
Free amino acids derive from three sources:
• Dietary proteins, which are absorbed as amino acids and peptides in the small intestine.
• De novo amino acid synthesis; we use carbon skeletons and the alpha-amino group of other amino acids to synthesize new ones.
• Protein turnover, the breakdown of cellular proteins.
AMINO ACID CONSUMPTION
Show that free amino acids are consumed in the following ways:
• Oxidation
Oxidation is the process in which excess amino acids are broken down into an alpha amino group and a carbon skeleton. The products of this reaction may enter the major following metabolic pathways: – The urea cycle: occurs in the liver and rids the body of nitrogen waste. – Energy production: amino acid carbon skeletons can enter glycolysis or the citric acid cycle. – Gluconeogenesis: occurs in the liver. – Storage: amino acids themselves cannot be stored; their carbon skeletons are converted to glycogen or fats if they aren't used immediately. Thus, oxidation can shunt excess amino acids into a number of different metabolic pathways.
• Synthesis of nitrogen-containing molecules
– Synthesis of nitrogen-containing molecules includes amino acids themselves (de novo amino acid synthesis),
3 / 5 neurotransmitters, nucleotides (purines and pyrimidines) and creatine, amongst others.
• Protein turnover
– Protein turnover both produces and consumes free amino acids. PROTEIN TURNOVER
Here, we draw-out protein turnover, which is the cyclic synthesis and breakdown of proteins in the body is called "Protein Turnover."
• Draw the amino acid pool and show that free amino acids can enter and leave it.
• Indicate that:
– Protein synthesis depletes the pool of free amino acids. – Protein breakdown in the body replenishes the pool. However, not all amino acids in the pool are the same. Amino acids that are present in excess can be used to synthesize and replenish the pool with amino acids that are scarce.
THE AMINO ACIDS
Let's learn the amino acids, now.
• Create a chart with two columns labeled essential and nonessential amino acids.
ESSENTIAL AMINO ACIDS 
Essential amino acids must be obtained from the diet, while nonessential amino acids can be synthesized endogenously.
• The essential amino acids include:
– Methionine, threonine, valine, phenylalanine, isoleucine, tryptophan, lysine, leucine (M, T, V, F, I, W, K, L). We can remember them by the following mnemonic: My Tall Vegan Friend Is Watering Kale Leaves. Note that histidine is sometimes considered an essential amino acid.
Glucogenic vs Ketogenic amino acids
• Glucogenic amino acids: methionine, threonine and valine
• Ketogenic amino acids: leucine and lysine.
Glucogenic and Ketogenic amino acids: phenylalanine, isoleucine, tryptophan. They can shunt into either pathway.* How do these essential amino acids enter gluconeogenesis or ketogenesis?
• Answer: they enter the liver, the primary site of metabolic integration!
Amino acid entrance into the liver.
• Draw a segment of the small intestine and indicate that dietary amino acids (both essential and nonessential) are absorbed, here.
• Show that they pass through the hepatic portal vein and enter the liver.
4 / 5 • Indicate that they may enter any of the three destinations that we illustrated in our amino acid pool diagram.
• Branched-chain amino acids (BCAA) [valine, leucine and isoleucine] immediately leave the liver and enter the circulation.
Branched-Chain Amino Acid (BCAA) Deamination
The liver cannot deaminate branched-chain amino acids – deamination is the first step in amino acid breakdown.
• Branched-chain amino acids travel to the muscle, which has a specific enzyme that can deaminate them.
Branched-Chain Amino Acid Functional Roles
Branched-chain amino acids play a number of important roles:
• They provide hydrophobic side chains in protein structure.
• They are used as a transport mechanism for nitrogen from the liver to the peripheral tissues.
Muscle tissues can use these amino acids to synthesize nonessential amino acids. NONESSENTIAL AMINO ACIDS
They include the "rest" of the amino acids.
• alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, histidine, proline, serine and tyrosine.
• All of them except for tyrosine are all glucogenic amino acids.
Knowing their molecular make-up, we might expect this from amino acids that can be readily synthesized from carbon- skeletons in the body.
• Tyrosine is a ketogenic amino acid.
As a clinical correlation write that some amino acids are considered conditionally essential amino acids because they cannot be synthesized under certain physiologic or pathophysiologic conditions.
• Infants and children up to 5 years old cannot synthesize:
– Arginine (Carbon precursors: Glutamate/Glutamine/Proline; Other Precursors: Ammonia, Aspartate) – Cysteine (Carbon precursor: Serine; Other Precursor: Methionine) – Histidine – Tyrosine Reference: J. Nutr. July 1, 2000 vol. 130 no. 7 1835S-1840S, The American Society for Nutritional Sciences
• Immunosuppressed patients or those with postoperative infections are often treated with arginine and glutamine supplements.
Reference: Am J Physiol Endocrinol Metab. 2011 Aug; 301(2): E264–E266. doi: 10.1152/ajpendo.00223.2011
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