What Are the 20 Standard Amino Acids Their Structure Types and Properties
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.
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FAQs on Standard Amino Acids Structure Classification and Functions
1. What are the standard amino acids?
The standard amino acids are the 20 α-amino acids that are genetically encoded and incorporated into proteins during translation. Each standard amino acid has a central (α) carbon bonded to:
- An amino group (–NH2)
- A carboxyl group (–COOH)
- A hydrogen atom (–H)
- A unique R group (side chain)
2. How many standard amino acids are there?
There are 20 standard amino acids that are directly encoded by the genetic code in most organisms. These amino acids are incorporated into proteins during ribosomal protein synthesis. Although additional amino acids like selenocysteine and pyrrolysine exist in some organisms, the universally recognized standard set for basic biochemistry is 20.
3. What is the general structure of a standard amino acid?
The general structure of a standard amino acid consists of an α-carbon bonded to four different groups: –NH2, –COOH, –H, and an R group. It can be represented as:
- H2N–CH(R)–COOH
4. What are the different types of standard amino acids?
The 20 standard amino acids are classified based on the properties of their R groups into nonpolar, polar, acidic, and basic types.
- Nonpolar (hydrophobic): e.g., alanine, valine, leucine
- Polar uncharged: e.g., serine, threonine, asparagine
- Acidic (negatively charged): aspartic acid, glutamic acid
- Basic (positively charged): lysine, arginine, histidine
5. What is a peptide bond and how is it formed?
A peptide bond is an amide linkage formed between the carboxyl group of one amino acid and the amino group of another. It forms through a condensation reaction (dehydration synthesis):
- –COOH + –NH2 → –CO–NH– + H2O
6. What is the difference between essential and nonessential amino acids?
The difference between essential and nonessential amino acids is that essential amino acids cannot be synthesized by the body and must be obtained from the diet.
- Essential amino acids: e.g., leucine, lysine, tryptophan
- Nonessential amino acids: e.g., alanine, aspartic acid, glutamic acid
7. Why are amino acids amphoteric?
Amino acids are amphoteric because they can act as both acids and bases due to the presence of –COOH and –NH2 groups.
- The carboxyl group can donate a proton (acidic behavior).
- The amino group can accept a proton (basic behavior).
8. What is the isoelectric point (pI) of an amino acid?
The isoelectric point (pI) is the pH at which an amino acid has no net electrical charge. At this pH, the amino acid exists predominantly as a zwitterion with equal positive and negative charges. For simple amino acids without ionizable side chains, pI is calculated as:
- pI = (pKa1 + pKa2) / 2
9. Are all standard amino acids chiral?
All standard amino acids except glycine are chiral because their α-carbon is bonded to four different groups.
- Glycine is achiral because its R group is –H, giving it two identical substituents.
- All other standard amino acids exist as L-isomers in proteins.
10. What are examples of standard amino acids and their side chains?
Examples of standard amino acids can be distinguished by their unique side chains (R groups).
- Glycine (Gly): R = –H
- Alanine (Ala): R = –CH3
- Serine (Ser): R = –CH2OH
- Aspartic acid (Asp): R = –CH2COOH
- Lysine (Lys): R = –(CH2)4NH2
