# 12.2 Amino Acids

Amino acids are the monomers of proteins and are an essential part of human growth, development, and health. There are a total of twenty amino acids, with Asparagine being the first to be discovered in 1806. Amino acids are all composed of a carboxyl group, an amino group, and a hydrogen atom bonded to the same carbon atom-the alpha carbon (Basic Structure, 2003). They differ from one another in their side chains, or the R groups bonded to the alpha carbon. The R groups of the different amino acids can differ in size and structure, as well as electric charge (acidic or basic), which influences the solubility of the amino acids in water (Nelson and Cox, 2013).
There are twenty amino acids that are generally found in proteins, and they are: Glycine, Alanine, Valine, Isoleucine, Leucine, Proline, Methionine, Phenylalanine, Tyrosine, Tryptophan, Serine, Cysteine, Threonine, Asparagine, Glutamine, Aspartic acid, Glutamic acid, Histidine, Lysine, and Arginine. Proteins are broken down using hydrolysis reactions in which a water molecule is added to the protein, and this results in individual amino acids being formed. Proteins are synthesized using dehydration reactions (condensation reaction), in which a molecule of water is removed and a peptide bond forms between the amino acids. When two amino acids are joined together, the resulting molecule is called a dipeptide. When three are joined together, the molecule is called a tripeptide (Whitney and Rolfes, 2015). As a chain of many amino acids forms and grows, the chain is called a polypeptide.
The human body needs and uses the different amino acids to make proteins to help the body: 1) Metabolize food, 2) Repair torn or damaged body tissues, 3) Grow, and 4) Perform other important body functions (Amino acids, 2013).
The twenty amino acids are separated into three groups: 1) Essential amino acids, 2) Nonessential amino acids, and 3) Conditional Amino Acids. Essential amino acids are the amino acids that the body cannot synthesize by itself or cannot make in adequate quantities. Instead it must get these amino acids through food obtained in the diet. There are a total of nine essential amino acids. These are: Valine, Isoleucine, Leucine, Threonine, Histidine, Lysine, Phenylalanine, and Tryptophan. The nonessential amino acids are those that the body can synthesize by itself and make in large enough quantities. There are 11 nonessential amino acids and they can be supplied by food in the diet but it is not a requirement. Nonessential amino acids can be produces as long as there is a supply of Nitrogen to produce the amino group of the amino acid and pieces of fat or carbohydrates to form the rest of the structure. Conditional amino acids are amino acids that are usually nonessential but become essential during times of stress, illness, or other special circumstances. One such illness is Phenylketonuria, an inherited disease in which the nonessential amino acid tyrosine becomes essential because the body cannot convert phenylalanine into tyrosine (Whitney and Rolfes, 2015).
There are several foods that one should eat to obtain all essential amino acids. These foods include: beans, chicken, lean meat, fish, nuts and seeds, peanut butter, tofu, lentils and many others (Protein in diet, 2013).
Citations: 1) Amino acids: MedlinePlus Medical Encyclopedia. (2013, February 18). Retrieved October 4, 2015. 2) Whitney, E., & Rolfes, S. (2015). Proteins. In Understanding Nutrition (14th ed., p. Chapter 6). 3) Proteins, Peptides & Amino Acids. (2013, May 5). Retrieved October 4, 2015. 4) Nelson, D., & Cox, M. (2013). Amino Acids, Peptides, and Proteins. In Lehninger principles of biochemistry (6th ed., pp. 76–77). New York: W.H. Freeman and Company. 5) Protein in diet: MedlinePlus Medical Encyclopedia. (2013, April 30). Retrieved October 4, 2015. 6) Basic Structure of an Amino Acid. (2003, August 25). Retrieved October 4, 2015.
According to the Merriam-Webster dictionary, an amino acid is defined as “an amphoteric organic acid containing the amino group NH2.” In simpler terms, amino acids are the building blocks of proteins. All amino acids are composed of an amino group (NH2), an acid group (COOH), a hydrogen atom, a central carbon atom, and a side group. The side group is what distinguishes all the different amino acids from each other.
There are 20 amino acids. Of the 20 amino acids, 11 are nonessential, meaning that they can be produced and synthesized by the body. The remaining nine are essential, which means that they must be obtained from food. The essential amino acids are either not produced at all by the body or are not produced in sufficient amounts. Some amino acids are conditionally essential, meaning that they are normally nonessential, but must be obtained from the diet under special circumstances (Whitney & Rolfes, 2015).
In order to be used in the metabolic pathway, amino acids must undergo deamination. Deamination means they lose their nitrogen-containing amino group, and the only thing left is the carbon skeleton. After this process, they are able to enter the TCA cycle (Whitney & Rolfes, 2015).
Citations: Whitney, Ellie & Rolfes, Sharon R. (2015). Understanding nutrition (14th ed.). Canada: Cengage Learning.
The structure of a basic amino acid consists of a central carbon (C), an amino group (NH2), am atom of hydrogen (H), and an acid group (CHO2). Unlike carbohydrates or lipids, amino acids have a varying side group, or R-group, which occupies the remaining bond on the carbon and differentiates the twenty amino acids from each other. This R-group can be rather simple, being a hydrogen atom on the amino acid glycine, or rather complex as it is on the amino acid phenylalanine which has a multi carbon-hydrogen R-group (Whitney/Rolfes, 2013).
Overall there are two types of amino acids regarding human nutrition, essential and nonessential. Nonessential amino acids are ones that the human body can produce by itself using bodily nitrogen and excess carbs and lipids for the remaining carbon, hydrogen, and oxygen pieces of the amino acid. There are eleven nonessential amino acids and they include alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. Most proteins usually contain nonessential amino acids but it is not a requirement for our diet. On the other hand, essential amino acids are required in the human diet as they cannot be synthesized by the body. There are nine essential amino acids including histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. It is not necessarily vital to eat essential and nonessential proteins at every meal but it is important to balance their intake throughout the day (National Library of Medicine, 2015). There are special cases though in which one must include a nonessential amino acid into their diet for specific reasons, making this amino acid conditionally essential. For people with a disease known as phenylketonuria for example, they must include the nonessential amino acid tyrosine in their diet as tyrosine is made from phenylalanine, and if there is a complication in absorbing phenylalanine, than tyrosine has become conditionally essential for that person (Whitney/ Rolfes, 2013). Initially in a newborn only five of the nonessential amino acids are actually nonessential while the rest are conditionally essential until the body develops enough so that it can create the remaining nonessential amino acids in an efficient manner.
A protein is made by amino acids being linked together during a condensation reaction forming a dipeptide, tripeptide, or a polypeptide. The type of the amino acids and the order in which they sequence is vital as this directly determines the shape and function of the resulting protein (Australian Nutrient Reference Values, 2014). The general sequence of the amino acids in the amino acid chain determines what’s called its primary structure. The weak electrical charges with that polypeptide then determine the way one part of the chain interacts with another, creating its secondary structure. The unique side group on each amino acid and the way this side group interacts with the surrounding area determines the polypeptide’s tertiary structure. Then, if this polypeptide’s function requires that it binds with one or more other polypeptide then that creates a multiple polypeptide structure, or a quaternary structure.
Citations: 1)Nutrient Reference Values. (n.d.). Retrieved December 12, 2015, from https://www.nrv.gov.au/nutrients/protein 2)Amino acids: MedlinePlus Medical Encyclopedia. (n.d.). Retrieved December 12, 2015, from https://www.nlm.nih.gov/medlineplus/ency/article/002222.htm 3)Whitney, E., & Rolfes, S. (2015). Proteins. In Understanding Nutrition (14th ed., p. Chapter 6).

## 12.2.1 Glucogenic

Amino acids can be classified on the basis of essentiality, polarity, hydrophobia and their fate within the body once consumed. Specifically, amino acids can be separated into two groups according to the futures of their carbon fragments following deamination. If a carbon fragment later becomes a component of glucose, the amino acid to which it belongs is called a glucogenic amino acid (Whitney, 2002, p. 183). Specifically, if an amino acid becomes a pyruvate or a pyruvate-associated molecule within the Krebs Cycle, it is considered glucogenic. The production of glucose from glucogenic amino acids through a process called gluconeogenesis (closely associated with the Krebs cycle) occurs in the liver. In the first step of this two-step process, glucogenic amino acids are converted to alpha keto acids. In the second, alpha keto acids become glucose (Schutz, 2011). There are thirteen known glucogenic amino acids in the human body. Some examples of glucogenic amino acids in humans are glycine (used in some pharmaceuticals and metal finishing), serine (important in biosynthesis of purine and pyrimidines), valine, and histidine (precursor for histamine and carnosine synthesis). There are also some amino acids which can be considered both glucogenic and ketogenic, including tyrosine (important role in photosynthesis and signal transduction processes), tryptophan (essential amino acid important in protein biosynthesis) and phenylalanine (used in norepinephrine and epinephrine synthesis).
Works Cited: 1. Whitney, E., & Rolfes, S. (2002). Understanding nutrition (9th ed., p. 183). Belmont, CA: Wadsworth. 2. Schutz, Y. (2011, March 1). Protein turnover, ureagenesis and gluconeogenesis. Retrieved December 3, 2015, from http://www.ncbi.nlm.nih.gov/pubmed/22139560
A glucogenic amino acid is one that can be converted to pyruvate and can provide glucose. Thus, proteins are a good source of glucose when carbohydrates are not available. According to King (2015), “all amino acids except lysine and leucine are at least partly glucogenic.” Amino acids that are made into glucose go through a process known as glucogenesis. Glucogenesis occurs mainly in the liver, but also in the kidneys under certain circumstances, such as starvation. When amino acids are not used to make glucose, they are used instead as an energy source for other body cells (Walker & Rolfes, 2015).
Citations: 1. King, Michael W. (2015). Amino acids and proteins: Introduction to amino acid metabolism. The Medical Biochemistry Page. Retrieved from http://themedicalbiochemistrypage.org/amino-acid-metabolism.php. 2. Whitney, Ellie & Rolfes, Sharon R. (2015). Understanding nutrition (14th ed.). Canada: Cengage Learning.

## 12.2.2 Ketogenic

If the carbon fragment within an amino acid later becomes a component of ketone bodies, fats or sterols rather than glucose as in glucogenic amino acids, the amino acid is ketogenic. However, there are also some ketogenic amino acids that can also be considered glucogenic depending on the fate of their carbon fragment (such as tyrosine, phenylalanine and tryptophan). By definition, ketogenic amino acids are those that can be degraded directly into acetyl CoA which is the precursor to ketone bodies (Noguchi, 2010). Only two known amino acids are exclusively ketogenic in the human body: leucine and lysine. Leucine is used in formation of sterols in muscle and adipose tissue whereas lysine is closely correlated to serotonin function. There are five amino acids known in the human body which are both ketogenic and glucogenic, including isoleucine (involved in gluconeogenesis and Acetyl CoA), phenylalanine, tryptophan, tyrosine and threonine (precursor to glycine). While there are no disease specifically associated with any of the ketogenic amino acids, there is some evidence which indicates that ketogenic essential amino acid replacement diets can decrease the effects of a liver ailment known as hepatic steatosis in mice (Xu, 2013).
Works Cited: Noguchi, Y., & Nishikata, N. (2010, August 1). Ketogenic Essential Amino Acids Modulate Lipid Synthetic Pathways and Prevent Hepatic Steatosis in Mice. Retrieved December 3, 2015, from http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0012057
Ketogenic amino acids are those that make acetyl CoA, but cannot make glucose. King (2015) states that “lysine and leucine are the only amino acids that are solely ketogenic.” Ketogenic amino acids are converted into ketone bodies. Ketone bodies are acidic compounds and are produced by the liver when carbohydrates are not available and fats do not completely break down. Ketone bodies are normally produced and used in small enough amounts that provide the brain cells with fuel. Keto acids are compounds that contain a carbonyl group. When there are too many keto acids in the body, blood pH drops, signaling that the body’s chemistry is not in balance. This is known as ketosis. Ketosis also causes a loss of appetite, which may be beneficial when food is unavailable. Once a person is able to eat again, appetite returns and the body leaves ketosis (Whitney & Rolfes, 2015).
Citations: 1. King, Michael W. (2015). Amino acids and proteins: Introduction to amino acid metabolism. The Medical Biochemistry Page. Retrieved from http://themedicalbiochemistrypage.org/amino-acid-metabolism.php. 2. Whitney, Ellie & Rolfes, Sharon R. (2015). Understanding nutrition (14th ed.). Canada: Cengage Learning.

## 12.2.3 Urea

Proteins from diet can be broken down in the liver or in muscle through several processes. These proteins are broken down into amino acids and those amino acids have to further dispose of their amino groups. This process can occur through transamination which is the removal of the amino group. This allows the resulting parts of the amino acid to be utilized for energy. However, these amino acids must be removed from the body because a build up of amino groups could be a potential toxin for body. Even in small amounts amino groups could be fatal towards an individual. Essentially, urea is created when proteins are broken down. The process by which the urea is safely removed from the body is called the Urea Cycle (also referred to as the ornithine cycle. The Urea cycle occurs in the liver and is the main method in removing excess nitrogen from the body. In addition, the Urea cycle works in unity with the citric acid cycle as well. This is accomplished through the exchanging different intermediates and substances needed for each reaction. Urea is typically removed from the body by way of blood. Urea travels through the body from the liver to the kidney. There the kidney filters out the Urea from blood and it excreted out of the body in urine. However, there are tests that can be performed to see the blood urea nitrogen (Chemistry).This test is very important to see if an individual's liver and kidney are healthy and functioning. If a person's blood nitrogen level is higher than normal it could mean that the liver or kidney are not functioning properly (Webmd) . Doctors will be able to tell if individual has disease in her kidney through these tests. Some of these diseases could be either acute kidney failure or end stage renal disease (Webmd). Urea is the main end product of nitrogen metabolism. Urea was first discovered by German chemist Friedrich Wohler in 1828 while he was trying to make ammonium cyanate from silver cyanide and ammonium chloride. However, by mistake he ended up making Urea. Organically, Urea is a carbamide which a compound that is made up of two amino groups that are linked by a carbonyl (Boswick). Some of the main qualities of Urea is that it does not smell, has no color, and is water soluble. The reason behind urea being soluble is that in water it can get displaced into ammonium ion and bicarbonate ion. The water solubility is why urea is excreted in urine because it is the most efficient process. Urea is also utilized outside the body as well. For example, Urea is one main components of fertilizer. In addition, Urea can be synthetically made to be utilized for plastic, glue, detergent, or even pesticides (Boswick). This illustrates that Urea could have many potential uses not just used in the body.
Ammonia is a highly toxic substance to humans. If not excreted from the body, its accumulation can lead to liver damage, and even death (Ophardt, 2003). Ammonia is cleared from the body via urea. Urea is made in the liver and is excreted in the kidneys as urine. Urea is the main excretion method of unused nitrogen. The more protein a person consumes, the more urea is produced. In order to keep urea in solution, water must also be consumed. Thus, a person who eats a lot of protein must also drink a lot of water. This will help to dilute and excrete urea. If a person on a high-protein diet doesn’t drink enough water, they may become dehydrated. Without extra water, the body uses its stored water to get rid of urea. This is why it is common to see water loss in those who eat a lot of protein. Losing water makes high-protein diets seem effective, but actually has no value to losing body fat (Whitney & Rolfes, 2015).
Citations:
* "Blood Urea Nitrogen." Web Md. N.p., n.d. Web. 2 Dec. 2015. <http://www.webmd.com/a-guides/blood-urea-nitrogen>.
* Boswick, Heleigh, and Lucy Oppenheimer. "What is Urea." Wisegeeks. N.p., n.d. Web. 2 Dec.2015.http://www.wisegeek.com/whatis-urea.htm.
* Ophardt, Charles E. (2003). Urea cycle. Virtual ChemBook. Retrieved from http://chemistry.elmhurst.edu/vchembook/633ureacycle.html.
* Whitney, Ellie & Rolfes, Sharon R. (2015). Understanding nutrition (14th ed.). Canada: Cengage Learning.