Introduction:
Amino acids are the basic building blocks of proteins that serve as the nitrogenous backbones for compounds like neurotransmitters and hormones in human body.
Proteins are long chains of amino acids.
Amino acid sequences are unique to each protein. Because of the sequence, the protein can take on a variety of shapes and function in the body.
Chemistry:
An amino acid is an organic chemical. Organic chemicals contain carbon-hydrogen bonds.
The basic structure of all amino acids is identical. A basic amino group, a carboxylic acid group, a hydrogen atom, and an R-group, or side-chain group, are all attached to a central carbon atom in each molecule. The chemical nature of each amino acid is determined by the R-group.
Fig: General chemistry of Amino Acid
The chemical composition of the amino acid determines how it interacts with other amino acids and its surroundings. The elements present in every amino acid are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) (CHON). In addition, sulfur (S) is present in the side chains of some of amino acids like cysteine and methionine, and selenium in the less common amino acid selenocysteine.
Proteins are composed when amino acids are linked together by peptide bonds. The protein is then folded into precise three-dimensional forms by the forces of other amino acids and the effects of their R-groups. Twenty different types of amino acids, encoded by universal genetic code, are naturally incorporated into polypeptides and are called proteinogenic or standard amino acids.
Classification:
Basesd on polarity
Amino acids with polar but uncharged R Group
- These amino acids are polar and have a pH of 7.0.
- E.g., Glycine, Serine, Threonine, Cysteine, Glutamine, Asparagine and Tyrosine; (GSTCENY)
Non-polar amino acids
- Amino acids with non-polar R group:
- These are hydrocarbons in nature, hydrophobic, have aliphatic and aromatic groups.
- E.g., Alanine, Leucine, Isoleucine, Valine, Methionine, Phenyl alanine, Tryptophan and Proline; (ALIVMFWP)
Polar amino acids with negatively charged R group
- Their side chain (-R Group) contain extra carboxyl group with a dissociable proton.
- E.g., Aspartic acid and Glutamic acid; (DE)
Polar amino acids with positively charged R group
- Their side chain has extra amino group interpreting basic nature to protein.
- E.g., Lysine, Arginine and Histidine; (LRH)
Based on functional (-R) groups
Simple amino acids: These have no functional group in their side chain.
Example: glycine, valine, alanine, leucine, isoleucine
Hydroxy amino acids: These have a hydroxyl group in their side chain.
Eg: serine, threonine
Sulfur containing amino acids: They possess sulfur in their side chain.
Eg: cysteine, methionine
Aromatic amino acids: They contain benzene ring in their side chain
Eg: phenylalanine, tyrosine
Heterocyclic amino acids:
These are the amino acids having a side chain ring which possess at least on atom other than carbon.
Eg: Tryptophan, histidine, proline
Acidic amino acids and their amides:
In their side chain, they have carboxyl groups. Amino acid derivatives in which one of the carboxyl groups has been replaced by an amide group.
Eg: Aspartic, Glutamic acid Asparagine, glutamine
Branched chain amino acids:
A branched-chain amino acid (BCAA) is an amino acid having aliphatic side-chains with a branch.
Eg: leucine, isoleucine, valine
Basic amino acids: They contain amino group in their side chain.
Eg: Lysine, Arginine
Imino acid: Amino acids containing a secondary amine group
Eg: Proline
Nutritional Classification
Essential (Indispensable) amino acids
Essential amino acids are amino acids that humans and other vertebrates cannot synthesis from metabolic intermediates.
Because the human body lacks the metabolic mechanisms to produce certain amino acids, they must be obtained from an external diet. These types of amino acids are very essential for the proper growth and development of the body. There are nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Nonessential (Dispensable) amino acids
Non-essential amino acids, also known as dispensable amino acids, are those that can be left out of a diet. Essential amino acids are the precursors for the synthesis of these amino acids in the human body. These amino acids are mostly synthesized from glucose.
Tissue growth and repair, immunological function, red blood cell creation, and hormone synthesis are all aided by these amino acids. Unlike necessary amino acids, however, if enough protein sources with essential amino acids are provided, a healthy body can produce these proteins.
The carbon skeleton of non-essential amino acids can be provided by cells in the body. These carbon skeletons derive from intermediates in the glycolytic pathway and the Citric Acid Cycle (Tricarboxylic Acid Cycle or Kreb’s Cycle). Enzymatically catalyzed transamination of preexisting amino acids can be used to introduce the alpha-amino group. Transaminases or aminotransferases are the enzymes involved during this metabolism.
Arginine, glutamine, tyrosine, cysteine, glycine, proline, serine, ornithine, alanine, asparagine, and aspartate are the 11 non-essential amino acids.
Conditional (semi-essentials) amino acids
These amino acids may be considered conditionally essential because the body cannot synthesize them in sufficient quantities during certain physiological periods of growth, including pregnancy, adolescent growth, or recovery from trauma. Conditional amino acids are usually not essential, except in times of illness and stress. Arginine, glutamine, tyrosine, cysteine, glycine, proline, serine, and ornithine are conditional amino acids.
Based on Metabolic fate ( Metabolism)
The carbon skeleton of amino acids can serve as a precursor for the synthesis of glucose (glycogenic) or fat (ketogenic) or both. From metabolic view point, amino acids it is broadly classified into:
Glycogenic amino acids
These amino acids can serve as precursors for the formation of glucose or glycogen. e.g., alanine, aspartate, glycine, methionine etc.
Ketogenic amino acids
Fat can be synthesized from these amino acids. Leucine and lysine are the only amino acids that are completely ketogenic.
Glycogenic and ketogenic amino acids
The four amino acids isoleucine, phenyl- alanine, tryptophan, tyrosine are precursors for synthesis of glucose as well as fat.
Sources:
- Amino acids are essential for a variety of biological and chemical activities in our bodies, including tissue building and repair, enzyme production and function, food digestion, and molecule transportation.
- Only a few amino acids can be synthesized by human bodies, thus the rest, known as essential amino acids, should always be acquired from protein-rich diets.
- A wide variety of meals include essential amino acids. Animal proteins, such as beef, chicken, and eggs, are excellent providers of amino acids. Animal proteins are the best for the human body to absorb and utilize.
- Complete proteins are foods that are made up of all nine essential amino acids. Beef, poultry, fish, eggs, dairy, soy, quinoa, and buckwheat are some of the foods that fall under this category. Incomplete proteins are foods that only contain some of the required amino acids. Nuts, seeds, beans, and grains are examples of these foods. In case of a vegetarian or vegan, it is highly recommended to eat a variety of incomplete proteins to acquire all nine essential amino acids.
- Amino acid supplements are consumed by some people to help them sleep better, feel better, and perform better in sporting events. However, before beginning any supplements, including amino acid supplements, it is always advisable to contact with the doctor.
Uncommon (Non-standard) Amino acids:
- The polarity, or distribution of electric charge, of the R group is used to classify amino acids as standard or nonstandard (side chain). Standard amino acids are the 20 (or 21) amino acids that serve as the building blocks of proteins.
- Nonstandard amino acids are standard amino acids that have been chemically changed after being integrated into a protein (posttranslational modification), as well as amino acids that occur naturally in living organisms but are not found in proteins.
- Carboxyglutamic acid, a calcium-binding amino acid residue located in the blood-clotting protein prothrombin, is one of the latter.
- Phosphorylation, in which a phosphate molecule is added to the hydroxyl part of the R groups of serine, threonine, and tyrosine, is the most important posttranslational alteration of amino acids in eukaryotic organisms (including humans). In the control of protein function and cell signaling, phosphorylation plays a crucial role.
Function:
- Amino acids have a wide range of applications. Amino acids have been discovered to be regulators of gene expression and protein phosphorylation cascades, in addition to being building blocks and cell signaling molecules.
- Amino acids are essential components as well as a source of energy for cell survival, regeneration, and growth.
- Amino acids are the building blocks for a variety of hormones and nitrogenous compounds (cytochromes, heme, hormones, melanin, neurotransmitters, nucleotides/nucleic acids and others) and their biological significance is incomparable.
- It is also involving in various cellular process that function in oxidative stress, signaling pathways, thyroid hormone metabolism, and protein folding.
- Functional amino acid supplements, such as arginine, cysteine, glutamine, leucine, proline, and tryptophan, are seen to be beneficial for a variety of health-related disorders, including intestinal dysfunction, obesity, diabetes, cardiovascular disease, metabolic disorders, and infertility, in individuals of all ages.
- Furthermore, several amino acids control essential metabolic processes that are required for maintaining, growing, reproducing, and defending against infection.
- Moreover, certain non-essential amnio acids such as D-dimethylglycine, Azaserine, N-Acetylcystein, Cycloserine or Gabapentin are useful as drugs.
- Amino acids also presence in bacterial cell wall that will maintain the structural and functional of cell.
- Athletes and sports enthusiasts use amino acids to strengthen muscle and decrease fat accumulation; unfortunately, cognitive adverse effects and renal damage have been linked to amino acid supplementation.
Deficiency:
- Specific amino acid deficiencies are uncommon, while some categories of amino acids can be insufficient in many diets.
- Impaired immunity, digestive problems, depression, fertility difficulties, reduced mental alertness, delayed growth in children, and a variety of other health problems can all be triggered by an amino acid insufficiency.
- Each of the necessary amino acids has a distinct function in the body, and depletion symptoms vary correspondingly.
- Despite this, high quantities of amino acids and the metabolites of their synthesis, such as ammonia and homocysteine, can lead to neurological dysfunction, oxidative stress, and cardiovascular disease.
References:
- Jood S, Kapoor AC, Singh R. Amino acid composition and chemical evaluation of protein quality of cereals as affected by insect infestation. Plant Foods Hum Nutr. 1995 Sep;48(2):159-67.
- Hou Y, Yin Y, Wu G. Dietary essentiality of “nutritionally non-essential amino acids” for animals and humans. Exp Biol Med (Maywood). 2015 Aug;240(8):997-1007.
- John E. Hall PhD, in Pocket Companion to Guyton and Hall Textbook of Medical Physiology, 2021.
- Nelson, D.L., Lehninger, A.L. and Cox, M.M., 2008. Lehninger principles of biochemistry. Macmillan.
- Satyanarayana, U., 2021. Biochemistry, 6e-E-book. Elsevier Health Sciences.
- Voet, D. and Voet, J.G., 2010. Biochemistry. John Wiley & Sons.
- www.khanacademy.org/…/chemistry-of-amino-acids-and-protein-structure