Introduction to Standard Amino Acids
Amino acids are organic compounds that contain the functional groups amino (–NH₂) and carboxyl (–COOH), as well as a side chain (R group) unique to each amino acid. Carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) are the four essential elements of amino acids, while other elements can be present in the side chains of some amino acids.
In this article standard, sigma amino acid standard and non standard amino acids are discussed in detail.
Standard Amino Acids Definition
What are the standard and non standard amino acids?
The polarity (that is, the distribution of electric charge) of the R group is one of the most useful ways to classify the regular (or common) amino acids.
Given below is the number of standard amino acids and also standard amino acid structure.
Twenty Standard Amino Acids
1. Non-Polar Amino Acid
Glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, and tryptophan are all members of Group I amino acids. These amino acids have either aliphatic or aromatic groups in their R groups. This makes them hydrophobic. Globular proteins fold into a three-dimensional form in aqueous solutions to bury these hydrophobic side chains in the protein interior.
A. Glycine
Glycine is the only amino acid that is not optically active and was the first to be isolated from a protein, in this case, gelatin (no d- or l-stereoisomers). When introduced into proteins, it is the most unreactive of the -amino acids due to its structural simplicity.
B. Leucine
In 1819, leucine was isolated from cheese, and in 1820, it was isolated in its crystalline form from muscle and wool. It was first synthesized in a laboratory in 1891. Given below is the amino acid standard structure.
C. Alanine
Alanine was first discovered in protein in 1875 and accounts for 30% of the residues in silk. Silk fibres are strong, stretch resistant, and flexible thanks to their low reactivity and clear, elongated structure with few cross-links.
D. Valine
After being isolated from albumin in 1879, the structure of valine was discovered in 1906. Mammalian proteins only contain the l-stereoisomer. Valine can be broken down into simpler compounds in the body, but in people with maple syrup urine disease, a defective enzyme prevents this from happening and can be fatal if left untreated.
E. Tryptophan
The structure of tryptophan was identified in 1907 after it was isolated from casein (milk protein), but only the l-stereoisomer appears in mammalian proteins.
F. Phenylalanine
In 1879, phenylalanine was isolated from a natural source (lupine sprouts) and chemically synthesized in 1882. The human body is normally capable of converting phenylalanine to tyrosine, but in people who have the hereditary disorder phenylketonuria (PKU), the enzyme responsible for this conversion is inactive.
G. Methionine
In 1922, methionine was isolated from the milk protein casein. Methionine is an essential sulfur source for a variety of body compounds, including cysteine and taurine. Methionine prevents fat accumulation in the liver and assists in the detoxification of metabolic wastes and contaminants, due to its sulfur content.
H. Isoleucine
In 1904, isoleucine was discovered in beet sugar molasses. The hydrophobic nature of isoleucine's side chain plays a key role in deciding the tertiary structure of proteins that contain it.
2. Polar Uncharged Amino Acids
Serine, cysteine, threonine, tyrosine, asparagine, and glutamine are all members of Group II amino acids. This group's side chains have a wide range of functional groups. Most, however, have at least one atom with electron pairs usable for hydrogen bonding to water and other molecules (nitrogen, oxygen, or sulfur).
A. Serine
Serine was first isolated in 1865 from silk protein. Humans can make serine from other metabolites, such as glycine, but mammalian proteins only contain the l-stereoisomer. Serine is required for the biosynthesis of several metabolites and is frequently required for the catalytic role of enzymes that contain it, such as chymotrypsin and trypsin.
B. Tyrosine
Tyrosine was isolated from the degradation of casein (a cheese protein) in 1846, after which it was synthesized in the lab and its structure determined in 1883. Humans can synthesize tyrosine from phenylalanine.
C. Glutamine
In 1883, glutamine was isolated from beet juice, then from a protein in 1932, and finally chemically synthesized the following year. Glutamine is the most abundant amino acid in the human body and serves a variety of functions.
D. Threonine
In 1935, threonine was isolated from fibrin and synthesized the following year. In mammalian proteins, only the l-stereoisomer appears, and it is relatively unreactive. Although it is involved in a variety of bacterial reactions, its metabolic function in higher animals, including humans, is unknown.
E. Asparagine
The first amino acid to be isolated from a natural source was asparagine, which was purified from asparagus juice in 1806.
F. Cysteine
Cysteine was first isolated from a urinary calculus in 1810 and from hooves in 1899, and it is abundant in the proteins of fur, hooves, and skin keratin. It was then chemically synthesized, and the structure was determined in 1903.
3. Acidic Amino Acids
Aspartic acid and glutamic acid are the two amino acids in this category. Each one has a carboxylic acid on its side chain, which makes it acidic (proton-donating). The three functional groups on these amino acids can ionize in an aqueous solution at physiological pH, resulting in an overall charge of 1. Aspartate and glutamate are the ionic forms of amino acids.
A. Aspartic Acid
Aspartic acid was discovered in 1868 and is present in animal proteins; however, only the l-stereoisomer participates in protein biosynthesis.
B. Glutamic Acid
In 1866, glutamic acid was isolated from wheat gluten, and in 1890, it was chemically synthesized. Only the l-stereoisomer is present in mammalian proteins, which humans can synthesize from the natural intermediate -ketoglutaric acid.
4. Basic Amino Acids
Arginine, histidine, and lysine are the three amino acids that make up this category. Each side chain is fundamental (i.e., can accept a proton). At physiological pH, both lysine and arginine have an average charge of +1). Ionic bonds form between the side chains of arginine and lysine, just as they do with aspartate and glutamate.
A. Arginine
When proteins are digested in humans, arginine is formed. The human body will then transform it into nitric oxide, a chemical that relaxes blood vessels.
Arginine has been proposed for the treatment of chronic heart disease, elevated cholesterol, impaired circulation, and high blood pressure due to its vasodilatory effects.
B. Lysine
In 1889, lysine was isolated from the milk protein casein, and its structure was discovered in 1902. Lysine is essential for the binding of enzymes to coenzymes and the proper functioning of histones.
Did You Know?
Non standard amino acids are those that have been chemically modified after being inserted into a protein (a process known as "post-translational modification"), as well as those that occur naturally in living organisms but are not present in proteins. Carboxyglutamic acid, a calcium-binding amino acid residue present in the blood-clotting protein prothrombin, is one of these modified amino acids (as well as in other proteins that bind calcium as part of their biological function). Collagen is the most abundant protein in vertebrates in terms of mass. 4-hydroxyproline and 5-hydroxylysine are modified versions of proline and lysine that make up a significant portion of collagen's amino acids.
Revising the concept of Standard Amino Acids
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The concept of Standard Amino Acids - Detailed Explanation and FAQs is quite important to learn for all the students. This concept teaches you about various amino acids and how they are used in different areas. By understanding the uses of amino acids, you can enhance your knowledge of chemistry and ensure good marks in the exam. Here are some reasons why the Standard Amino Acids - Detailed Explanation and FAQs is important to learn:
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FAQs on Standard Amino Acids
1. What is a standard amino acid?
A standard amino acid is one of the 20 genetically coded organic compounds that serve as the primary building blocks of proteins in living organisms. Each standard amino acid features a central carbon atom (the alpha-carbon) attached to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a unique side chain known as the R-group, which determines its specific properties.
2. How are the 20 standard amino acids classified based on their R-groups?
Standard amino acids are classified based on the chemical nature of their side chains (R-groups), which dictates how they interact within a protein. The main categories are:
- Nonpolar, Aliphatic: Have hydrophobic side chains. Examples include Glycine, Alanine, Valine, Leucine, Isoleucine, and Proline.
- Aromatic: Contain an aromatic ring in their side chain. Examples are Phenylalanine, Tryptophan, and Tyrosine.
- Polar, Uncharged: Possess hydrophilic side chains that can form hydrogen bonds. Examples include Serine, Threonine, Cysteine, Asparagine, and Glutamine.
- Acidic (Negatively Charged): Have a carboxyl group in their side chain. Examples are Aspartic acid and Glutamic acid.
- Basic (Positively Charged): Contain an amino group in their side chain. Examples are Lysine, Arginine, and Histidine.
3. What is the importance of distinguishing between essential and non-essential amino acids?
The distinction is crucial for understanding nutrition and metabolic health. Essential amino acids cannot be synthesised by the human body and must be obtained from dietary sources like meat, eggs, and beans. There are nine of them, including Leucine and Valine. Non-essential amino acids can be produced by the body, so their dietary intake is not as critical. Understanding this helps in designing balanced diets and diagnosing nutritional deficiencies.
4. Why are the standard amino acids referred to as α-amino acids?
They are called α (alpha)-amino acids because the amino functional group (–NH₂) is attached to the alpha-carbon. The alpha-carbon is the first carbon atom bonded directly to the main functional group, which in this case is the carboxyl group (–COOH). This specific structural arrangement is fundamental to the formation of peptide bonds and, subsequently, proteins.
5. Explain the concept of a Zwitterion in the context of amino acids.
A Zwitterion is a molecule that contains both a positive and a negative charge but has an overall neutral charge. In a neutral aqueous solution, an amino acid exists in this form because the acidic carboxyl group (–COOH) donates a proton and becomes a negative carboxylate ion (–COO⁻), while the basic amino group (–NH₂) accepts that proton to become a positive ammonium ion (–NH₃⁺). This dipolar nature explains many of their properties, such as their high melting points and solubility in water.
6. What makes glycine unique compared to the other 19 standard amino acids?
Glycine is unique because its R-group is simply a single hydrogen atom. This has two major consequences:
- It is the smallest and most flexible amino acid, allowing it to fit into tight spaces within a protein's structure.
- It is the only standard amino acid that is not optically active. Because its central carbon is bonded to two identical hydrogen atoms, it lacks a chiral centre. All other standard amino acids are chiral and exist in L- and D-forms (though only L-forms are used in proteins).
7. How do the sulfur-containing amino acids, Cysteine and Methionine, contribute to protein structure and function?
Methionine and Cysteine have unique roles due to the presence of sulfur. While Methionine often serves as an initial amino acid in protein synthesis, Cysteine is vital for structural stability. The side chains of two Cysteine residues can form a covalent disulfide bond (–S–S–). This strong bond acts as a 'staple', locking parts of the protein together and playing a critical role in defining the final three-dimensional tertiary and quaternary structures of many enzymes and structural proteins.
8. Are there any known 'standard' amino acids beyond the main 20?
While the 20 amino acids are the universally recognised set, two others are sometimes called the 21st and 22nd proteinogenic amino acids because they can be incorporated into proteins during translation in some organisms. These are Selenocysteine (Sec) and Pyrrolysine (Pyl). They are considered special cases as they are encoded by codons that typically signal for translation to stop and are not found in all proteins or all forms of life.
