Amino acids



Amino acids are organic compounds that contain amine (–NH2) and carboxyl (–COOH) functional groups, along with a side chain (R group) specific to each amino acid.


The key elements of an amino acid are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), although other elements are found in the side chains of certain amino acids. 


There are about 500 naturally occurring amino acids.


Only 20 amino acids appear in the genetic code.


AAs are the structural units that make up proteins. 

Used in synthesis of proteins including hormones, enzymes, and growth factors.

Used as an energy source.

Consist of carboxylic acid, an amine group, and a characteristic functional side group.

Acidic amino acids include aspartic acid and glutamic acid and they contain an additional carboxylic acid group and are negatively charged.

Basic amino acids include arginine, lysine, and histidine which are polar and hydrophilic.

AAs join together to form short polymer chains called peptides or longer chains called either polypeptides or proteins.

AA  are classified according to the core structural functional groups’ locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids and other categories relate to polarity, pH level, and side chain group type such as aliphatic, acyclic, aromatic, containing hydroxyl or sulfur.

AA in n the form of proteins, are the second-largest component of human muscles and other tissues, after water.


AA’s have role as residues in proteins, and participate in neurotransmitter transports and biosynthesis.


AAs have important proteinogenic and non-proteinogenic biological functions. 


Glutamic acid and gamma-aminobutyric acid (“GABA”) are, respectively, the main excitatory and inhibitory neurotransmitters of the brain.


Proline is synthesized to hydroxyproline a major component of the connective tissue collagen.


The amino acid glycine is a biosynthetic precursor to RBC porphyrins.


Carnitine AA is used in lipid transport.


There are 9 proteinogenic amino acids essential for humans because they cannot be produced from other compounds by the human body and so must be taken in as food. 


Others AAs  are conditionally essential for certain ages or medical conditions. 


Amino acids are commonly used in nutritional supplements, fertilizers, feed, and food technology. 


Industrial uses include the production of drugs, and biodegradable plastics.


The carbon atom next to the carboxyl group is called the α–carbon, referred to as alpha amino acids.


Alpha amino acids are the most common form found in nature, occurring in the L-isomer. 


All alpha amino acids but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other.


L-amino acids represent all of the amino acids found in proteins during translation in the ribosome.


D-amino acids are found in some proteins produced by enzyme posttranslational modifications after translation and translocation to the endoplasmic reticulum.


AAs  are components of the peptidoglycan cell walls of bacteria.


D-serine may act as a neurotransmitter in the brain.


Amino acids are designated as α- when the nitrogen atom is attached to the carbon atom adjacent to the carboxyl group.


Amino acids with the sub-structure N–C–C–CO2 are classified as β- amino acids. 


Amino acids are usually classified by the properties of their side chain into four groups. 


Side  chains make an amino acid a weak acid or a weak base, and a hydrophile if the side chain is polar or a hydrophobe if it is nonpolar.


Branched-chain amino acids have aliphatic side chains that are linear; these are leucine, isoleucine, and valine. 


Proline is the only proteinogenic amino acid whose side-group links to the α-amino group.


In aqueous solution amino acids exist in two forms, the molecular form and the zwitterion form in equilibrium with each other. 


All amino acids contain amine and carboxylic acid functional groups, making them amphiprotic.


Each amino acid within a polymer chain is attached to two neighboring amino acids. 


The process of making proteins encoded by DNA/RNA genetic material is called translation.


Translation involves the step-by-step addition of amino acids to a growing protein chain by a ribosome.


The order in which the amino acids are added is read through the genetic code from an mRNA template, which is an RNA copy of one of the genes.


The 22 amino acids are naturally incorporated into polypeptides and are called proteinogenic or natural amino acids.


Aside from the 22 proteinogenic amino acids, many non-proteinogenic amino acids are known. 


Non-proteinogenic amino acids are not found in proteins or are not produced directly and in isolation by standard cellular mechanisms.


Non-proteinogenic amino acids that are found in proteins are formed by post-translational modification.


Post-translational modification occurs after translation during protein synthesis, and are often essential for the function or regulation of a protein. 


Some non-proteinogenic amino acids are not found in proteins. 


Non-proteinogenic amino acids often occur as intermediates in the metabolic pathways for standard amino acids.


The 20 amino acids that are encoded directly by the codons of the universal genetic code are called standard amino acids. 


Glutamate and glutamine are the most frequent AA in food at over 10%, while alanine, glutamine, and glycine are the most common in blood.


When taken into the body from the diet, the 20 standard amino acids are used to synthesize proteins, other biomolecules, or are oxidized to urea and carbon dioxide as a source of energy.


The oxidation pathway removes of the amino group which feeds  into the urea cycle, and transamidation allows entry in to  the citric acid cycle.


Glucogenic amino acids can also be converted into glucose, through gluconeogenesis.

Out of the twenty standard protein-producing amino acids, nine cannot be endogenously synthesized by humans: phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.


The human body cannot synthesize them from other compounds at the level needed for normal growth.


Essential amino acids they must be obtained from food.

Adequate dietary intake of amino acids is essential for cellular responses, since amino acids and synthesize proteins are involved in diverse physiological functions, tissue, building, and organ development.

When intake and endogenous production of amino acids and proteins are less than expenditures of these nutrients, negative protein balance, ensues leading to catabolism and compromising of health and growth.

Cysteine, tyrosine, and arginine are considered semiessential amino acids, and taurine a semiessential aminosulfonic acid in children. 


Dietary exposure to amino acid BMAA has been linked to human neurodegenerative diseases, including ALS.


Many amino acids derived from food protein promote the activation of mTORC1 and increase protein synthesis.


Resistance training stimulates muscle protein synthesis (MPS) for a period of up to 48 hours following exercise, and the ingestion of a protein-rich meal at any point during this period augments the exercise-induced increase in muscle protein synthesis.


Non-protein amino acids act  as metabolic intermediates, such as in the biosynthesis of the neurotransmitter gamma-aminobutyric acid (GABA). 


Many amino acids synthesize other molecules: 


Tryptophan is a precursor of the neurotransmitter serotonin.


Tyrosine and its precursor phenylalanine are precursors of the catecholamine neurotransmitters dopamine, epinephrine and norepinephrine and various trace amines.


Phenylalanine is a precursor of phenethylamine and tyrosine in humans. 


Glycine is a precursor of porphyrins such as heme.


Arginine is a precursor of nitric oxide.


Ornithine and S-adenosylmethionine are precursors of polyamines.


Aspartate, glycine, and glutamine are precursors of nucleotides.


However, not all of the functions of other abundant nonstandard amino acids are known.


Amino acids are used in industry, as additives to animal feed.


The food industry is also a major consumer of amino acids: glutamic acid, which is used as a flavor enhancer, and aspartame as a low-calorie artificial sweetener.


Amino acid chelating ability is used in fertilizers for agriculture to facilitate the delivery of minerals to plants, to prevent deficiencies from occurring and improving the overall health of the plants.


Amino acids derivatives are used in pharmaceutical industry: 5-hydroxytryptophan, dihydroxyphenylalanine.


Amino acids have been considered as components of biodegradable polymers, with applications as environmentally friendly packaging and in medicine in drug delivery and the construction of prosthetic implants, and disposable diapers.


The commercial production of amino acids os buy mutant bacteria that overproduce individual amino acids.


Some amino acids are produced by enzymatic conversions of synthetic intermediates. 


Nonstandard amino acids are usually formed through modifications to standard amino acids: homocysteine is formed through the transsulfuration or by demethylation of methionine, 


hydroxyproline is made by a post translational modification of proline.


Microorganisms synthesize many uncommon amino acids. 


Amino acids undergo many reactions:


Peptide bond formation-two joined amino acids are called a dipeptide.


A dipeptide is the result of the condensation of two amino acids to form a dipeptide through a peptide bond: 


the amine and carboxylic acid groups of amino acids can react to form amide bonds, one amino acid molecule can react with another and become joined through an amide linkage. 


Amino acid polymerization is what creates proteins. 


All proteins made by ribosomes are synthesized starting at their N-terminus and moving toward their C-terminus.


Some peptides are synthesized by specific enzymes. 


Amino acids can be classified according to the properties of their main products: 


Glucogenic products having the ability to form glucose by gluconeogenesis 


Ketogenic, with the products not having the ability to form glucose. 


Amino acids are catabolized into both glucogenic and ketogenic products.


Amino acids pass out of organelles and cells into blood circulation via amino acid transporters.


Degradation of an amino acid, occurs  in the liver and kidneys.


Degradation involves deamination and transaminases.


The amino group is removed through the urea cycle and is excreted in the form of urea. 


Amino acid degradation can produce uric acid or ammonia as well. 


Amino acids can be classified according to their properties. Important factors are charge, hydrophilicity or hydrophobicity, size, and functional groups.


These properties influence protein structure and protein–protein interactions. 


Water-soluble proteins tend to have their hydrophobic residues (Leu, Ile, Val, Phe, and Trp) buried in the middle of the protein.


Hydrophilic side chains are exposed to the aqueous solvent. 


The outer rings of exposed hydrophobic amino acids anchor them into the lipid bilayer. 


Proteins that have to bind to positively charged molecules have surfaces rich with negatively charged amino acids like glutamate and aspartate.


Proteins binding to negatively charged molecules have surfaces rich with positively charged chains like lysine and arginine.


Many proteins undergo posttranslational modifications, whereby additional chemical groups are attached to the amino acid side chains. 


Posttranslational modifications can produce hydrophobic lipoproteins, or hydrophilic glycoproteins.



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