Chemistry And Structure Of Amino Acids (PDF)

**DAVID UMAHI FEDERAL UNIVERSITY OF HEALTH SCIENCE, UBURU** DEPARTMENT OF MEDICAL BIOCHEMISTRY DR. OKORO CHUKWUEMEKA OGBONNA CHEMISTRY AND STRUCTURE OF AMINO ACID **Amino acids share common structural features** Amino acids are the structural units or building blocks of proteins. Each AA is a n...

**DAVID UMAHI FEDERAL UNIVERSITY OF HEALTH SCIENCE, UBURU** DEPARTMENT OF MEDICAL BIOCHEMISTRY DR. OKORO CHUKWUEMEKA OGBONNA CHEMISTRY AND STRUCTURE OF AMINO ACID **Amino acids share common structural features** Amino acids are the structural units or building blocks of proteins. Each AA is a nitrogenous compound having an acidic carboxyl group (-COOH) and a basic amino group (-NH2), a hydrogen atom and a distinctive 'R' group or side chain all attached by covalent bonds to a carbon atom which is called an α-carbon. All AA except glycine have an asymmetric carbon resulting in 2 optically active forms-"D" and "L" forms. Only the "L" forms" are constituents of proteins. Amino acids at neutral pH are dipolar or zwitter ions. Proline and hydroxyproline have an imino group (NH) with a cyclic structure and are called imino acids. The specific order of amino acids in a protein is known as its primary structure. It is this sequence that determines the three-dimensional architecture of a protein. All amino acids are composed of an **amino group** (-NH2 ), a **carboxylic acid group** (-COOH), and an intervening **carbon** atom to which these two groups are connected. The intervening carbon atom is the alpha (α) carbon. For 19 of the 20 amino acids, an additional chemical group, known as an R group or side chain, is attached to the α carbon. The twentieth amino acid, glycine, has two hydrogen atoms connected to the α carbon instead of an R group and a single hydrogen atom. The unique chemical and structural properties of each amino acid are determined by the identity of the R group. Side chains (R) varying in size, shape, charge, hydrogen bonding capacity and chemical reactivity give rise to 20 difeerent AA which are found in proteins. Based on nature of the attached R group these AA may be broadly cassified as- Non-polar and aliphatic, Polar and uncharged, Negatively charged, Positively charged and Aromatic in nature. **Amino acids are chiral** A key feature of amino acids is that the α carbon is chiral. When a carbon atom is bound to four unique groups, it creates a chiral center (also known as a stereocenter). Simply put, a chiral molecule is one that cannot be superimposed with its mirror image. The configuration, or stereochemistry, of a chiral center is binary; a chiral center can either have one configuration or the mirror image of that configuration. Although chirality may seem like an abstract concept, it is not. You are surrounded by chiral objects; many macroscopic structures and most molecules in your body, large and small, are chiral. For example, your feet are chiral, which is why you cannot wear your left shoe on your right foot. Your hands are chiral, which is why if you are left-handed it is hard to use most scissors, which are designed for right-handed people simply because the majority of people happen to be right-handed. The two stereoisomers for each of the 19 chiral amino acids are denoted as D and L. Only the L-stereoisomer is used in nature to construct proteins. While this may seem arbitrary, remember that stereoisomers are differently shaped, distinct structures. For example, imagine a hundred amino acid-long protein in which each amino acid could be either D or L. If so, then there would be 2^100^ possible versions of the same protein. All of these proteins would have wildly different structures that would fold in unpredictable ways. **ESSENTIAL AND NON-ESSENTIAL AMINO ACIDS** Amino acids can be called the "building blocks" of protein and are an important part of every human body. There are 20 different amino acids -- nine of which are called "essential" and 11 of which are labeled as "non-essential." The human body needs all 20 of these amino acids, in varying degrees, to be healthy and fully functional. All 20 have distinct chemical structures and are used for different roles -- such as forming neurotransmitters, forming hormones and producing energy. But their primary role is to build proteins. Protein is part of every single cell in the human body and is essential to the body's functioning. Protein helps build and repair tissues like skin and muscle, and it helps produce antibodies and insulin. From only 20 amino acids, the body is able to generate many thousands of unique proteins with different functions. Each one of these proteins contains between 50 and 2000 amino acids, connected in varying sequences. After all of these amino acids are joined together, they are folded and twisted to make a specific shape. This unique shape is the determining factor for what the protein does for the body **Essential Amino Acids** Essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized de novo by the organism at a rate as per its demand, and thus must be supplied in its diet. These are the nine amino acids that cannot be create on its own, and which must obtain by eating various foods. Adults need to eat foods that contain the following nine amino acids phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. Instead of storing up a supply of the essential acids, the body uses them to create new proteins on a regular basis. Therefore, the body needs a continuous supply of these amino acids to stay healthy. **Non-Essential Amino Acids** The other type is the non-essential amino acid,11 of which exist and are synthesized by the body- alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. Thus, although they are an important part of building proteins, they do not need to be included in an everyday diet. Six of these non-essential acids are also known as "conditional," meaning that the body may not be capable of producing enough of them when presented with substantial stress or illness These are arginine, cysteine, glycine, glutamine, proline, and tyrosine.. The limiting amino acid is the essential amino acid found in the smallest quantity in the foodstuff, most plant-based foods have a limiting amino acid. Essential Non-essential Conditionally --- ------------------- ------------------- --------------- 1 Histidine (H) Alanine (A) Arginine (R) 2 Isoleucine (I) Aspartic acid (D) Cysteine (C) 3 Leucine (L) Asparagine (N) Glutamine (Q) 4 Lysine (K) Glutamic acid (E) Glycine (G) 5 Methionine (M) Serine (S) Proline (P) 6 Phenylalanine (F) Tyrosine (Y) 7 Threonine (T) 8 Tryptophan (W) 9 Valine (V **Specific amino acids give rise to specialized products**, e.g. Tyrosine forms hormones such as thyroid hormones, (T3, T4), epinephrine and norepinephrine and a pigment called melanin. Tryptophan can synthesize a vitamin called niacin. Glycine, arginine and methionine synthesis creatine. Glycine and cysteine help in synthesize of Bile salts. Glutamate, cysteine and glycine synthesis glutathione. Histidine changes to histamine on decarboxylation. Serotonin is formed from tryptophan. Glycine is used for the synthesis of haem. Pyrimidines and purines use several amino acids for their synthesis such as aspartate and glutamine for pyrimidines and glycine, aspartic acid, Glutamine and serine for purine synthesis. (c) Some amino acids such as glycine and cysteine are used as detoxicants of specific substances. \(d) Methionine acts as "active" methionine (S-adenosylmethionine) and transfers methyl group to various substances by transmethylation. \(e) Cystine and methionine are sources of sulphur Complete proteins contain a balanced set of essential amino acids for humans. Animal sources such as meat, poultry, eggs, fish, milk, and cheese provide all of the essential amino acids. Near-complete proteins are also found in some plant sources such as quinoa. The net protein utilization is affected by the limiting amino acid content (the essential amino acid found in the smallest quantity in the foodstuff), and somewhat affected by utilization of essential amino acids in the body. It is therefore important to mix foodstuffs that have different weaknesses in their essential amino acid distributions. This limits the loss of nitrogen through deamination and increases overall net protein utilization. Thus some of the most efficient pairings include sweet corn/tomatoes, apple/coconut and sweet corn/cherry. Certain traditional combinations of foods, such as corn and beans, or beans and rice, contain the essential amino acids necessary for humans in adequate amounts. proteins are long chains of covalently connected amino acids. The covalent linkage between two amino acids is called a peptide bond, hence the name polypeptide. Polypeptide chains are directional, and the two ends of the chain feature different chemical groups. At one end is a free amino group, and at the other end is a free carboxylic acid group. The end of the polypeptide chain with a free amino group is the amino terminus, which is frequently written as "N-terminus" or "NH3+-terminus." The end with a free carboxylic acid group is the carboxy terminus, which is frequently written as "C-terminus" or "COO−-terminus." When the amino acid sequence of a protein is specified, it must indicate the N-terminal-to-C-terminal directionality; reversing the directionality indicates a very different protein sequence! By convention, amino acid sequences are typically written left to right from amino to carboxy terminus. **Clinical Significance** Studies found that certain amino acids are \"conditionally essential,\" depending on the subject\'s metabolic state. For example, although a healthy adult may be able to synthesize tyrosine from phenylalanine, a young child may not have developed the required enzyme (phenylalanine hydroxylase) to perform this synthesis, and so they would be unable to synthesize tyrosine from phenylalanine, making tyrosine an essential amino acid under those circumstances. This concept also appears in different disease states. Basically, deviations from a standard healthy adult\'s metabolic state may place the body in a metabolic state that requires more than the standard-essential amino acids to be nitrogen balance. In general, the optimal ratio of essential amino acids and nonessential amino acids requires a balance dependent on physiological needs that differs between individuals. Finding the optimal ratio of amino acids in total parenteral nutrition for liver or kidney disease is a good example of different physiological states requiring different nutrient intakes. Therefore, the terms \"essential amino acid\" and \"nonessential amino acids\" may be misleading since all amino acids may be necessary to ensure optimal health. During states of inadequate intake of essential amino acids such as vomiting or low appetite, clinical symptoms may appear. These symptoms may include depression, anxiety, insomnia, fatigue, weakness, growth stunting in the young, etc. These symptoms are mostly caused by a lack of protein synthesis in the body because of the lack of essential amino acids. Required amounts of amino acids are necessary to produce neurotransmitters, hormones, the growth of muscle, and other cellular processes. These deficiencies are usually present in poorer parts of the world or elderly adults with inadequate care. Kwashiorkor and marasmus are examples of more severe clinical disorders caused by malnutrition and inadequate intake of essential amino acids. Kwashiorkor is a form of malnutrition characterized by peripheral edema, dry peeling skin with hyperkeratosis and hyperpigmentation, ascites, liver malfunction, immune deficits, anemia, and relatively unchanged muscle protein composition. It results from a diet with insufficient protein but adequate carbohydrates. Marasmus is a form of malnutrition characterized by wasting caused by inadequate protein and overall inadequate caloric intake.