Classifications of Amino Acids
Experts classify amino acids based on a variety of features, including whether people can acquire them through diet. Accordingly, scientists recognize three amino acid types:
1. Nonessential
2. Essential
3. Conditionally essential
However, the classification as essential or nonessential does not actually reflect their importance, as all 20 amino acids are necessary for human health.
Eight of these amino acids are essential (or indispensable) and cannot be produced by the body. They are:
• Leucine
• Isoleucine
• Lysine
• Threonine
• Methionine
• Phenylalanine
• Valine
• Tryptophan
Histidine is an amino acid that is categorized as semi-essential since the human body doesn't always need it to properly function; therefore, dietary sources of it are not always essential. Meanwhile, conditionally essential amino acids aren't usually required in the human diet, but do become essential under certain circumstances.
Finally, nonessential amino acids are produced by the human body either from essential amino acids or from normal protein breakdowns. Nonessential amino acids include:
• Asparagine
• Alanine
• Arginine
• Aspartic acid
• Cysteine
• Glutamic acid
• Glutamine
• Proline
• Glycine
• Tyrosine
• Serine
An additional amino acids' classification depends upon the side chain structure, and experts recognize these five as:
• Cysteine and Methionine (amino acids containing sulfur)
• Asparagine, Serine, Threonine, and Glutamine (neutral amino acids)
• Glutamic acid and Aspartic acid (acidic); and Arginine and Lysine (basic)
• Leucine, Isoleucine, Glycine, Valine, and Alanine (aliphatic amino acids)
• Phenylalanine, Tryptophan, and Tyrosine (aromatic amino acids)
One final amino acid classification is categorized by the side chain structure that divides the list of 20 amino acids into four groups - two of which are the main groups and two that are subgroups. They are:
1. Non-polar
2. Polar
3. Acidic and polar
4. Basic and polar
For example, side chains having pure hydrocarbon alkyl or aromatic groups are considered non-polar, and these amino acids are comprised of Phenylalanine, Glycine, Valine, Leucine, Alanine, Isoleucine, Proline, Methionine, and Tryptophan. Meanwhile, if the side chain contains different polar groups like amides, acids, and alcohols, they are classified as polar. Their list includes Tyrosine, Serine, Asparagine, Threonine, Glutamine, and Cysteine. If the side chain contains a carboxylic acid, the amino acids in the acidic-polar classification are Aspartic Acid and Glutamic Acid. Furthermore, if the side chain consists of a carboxylic acid and basic-polar, these amino acids are Lysine, Arginine, and Histidine.
Properties of Amino Acids
The properties of α-amino acids are complex, yet simplistic in that every molecule of an amino acid involves two functional groups: carboxyl (-COOH) and amino (-NH2).
As well, each molecule contains a side chain or an R group. And while alanine is an example of a standard amino acid (which is used in the biosynthesis of proteins), each R group has very different properties and functions.
Amino acids are crystalline solids which have the capacity to dissolve in water. Meanwhile, they only dissolve sparingly in organic solvents, and the extent of their solubility depends on the size and nature of the side chain. Amino acids feature very high melting points - up to 200-300°C with other properties varying for each particular amino acid.
Amino acids are classified in a variety of ways, but another of these is aromatic and aliphatic where aromatic is a special type of ring-shaped molecule, and it is characterized by an unusual stabilizing property. Aliphatic is non-aromatic.
Amino acids tend to evolve slower than DNA. However, changes in DNA do not affect amino acid properties or functions.
Group I: Nonpolar amino acids
Group I amino acids are glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, and tryptophan. The R groups of these amino acids have either aliphatic or aromatic groups. This makes them hydrophobic (“water fearing”). In aqueous solutions, globular proteins will fold into a three-dimensional shape to bury these hydrophobic side chains in the protein interior. The chemical structures of Group I amino acids are:
Isoleucine is an isomer of leucine, and it contains two chiral carbon atoms. Proline is unique among the standard amino acids in that it does not have both free α-amino and free α-carboxyl groups. Instead, its side chain forms a cyclic structure as the nitrogen atom of proline is linked to two carbon atoms. (Strictly speaking, this means that proline is not an amino acid but rather an α-imino acid.) Phenylalanine, as the name implies, consists of a phenyl group attached to alanine. Methionine is one of the two amino acids that possess a sulfur atom. Methionine plays a central role in protein biosynthesis (translation) as it is almost always the initiating amino acid. Methionine also provides methyl groups for metabolism. Tryptophan contains an indole ring attached to the alanyl side chain.
Group II: Polar, uncharged amino acids
Group II amino acids are serine, cysteine, threonine, tyrosine, asparagine, and glutamine. The side chains in this group possess a spectrum of functional groups. However, most have at least one atom (nitrogen, oxygen, or sulfur) with electron pairs available for hydrogenbonding to water and other molecules. The chemical structures of Group II amino acids are:
Two amino acids, serine and threonine, contain aliphatic hydroxyl groups (that is, an oxygen atom bonded to a hydrogen atom, represented as −OH). Tyrosine possesses a hydroxyl group in the aromatic ring, making it a phenol derivative. The hydroxyl groups in these three amino acids are subject to an important type of posttranslational modification: phosphorylation (see below Nonstandard amino acids). Like methionine, cysteine contains a sulfur atom. Unlike methionine’s sulfur atom, however, cysteine’s sulfur is very chemically reactive (see below Cysteine oxidation). Asparagine, first isolated from asparagus, and glutamine both contain amide R groups. The carbonyl group can function as a hydrogen bond acceptor, and the amino group (NH2) can function as a hydrogen bond donor.
Group III: Acidic amino acids
The two amino acids in this group are aspartic acid and glutamic acid. Each has a carboxylic acid on its side chain that gives it acidic (proton-donating) properties. In an aqueous solution at physiological pH, all three functional groups on these amino acids will ionize, thus giving an overall charge of −1. In the ionic forms, the amino acids are called aspartate and glutamate. The chemical structures of Group III amino acids are
The side chains of aspartate and glutamate can form ionic bonds (“salt bridges”), and they can also function as hydrogen bond acceptors. Many proteins that bind metal ions (“metalloproteins”) for structural or functional purposes possess metal-binding sites containing aspartate or glutamate side chains or both. Free glutamate and glutamine play a central role in amino acid metabolism. Glutamate is the most abundant excitatory neurotransmitter in the central nervous system.
Group IV: Basic amino acids
The three amino acids in this group are arginine, histidine, and lysine. Each side chain is basic (i.e., can accept a proton). Lysine and arginine both exist with an overall charge of +1 at physiological pH. The guanidino group in arginine’s side chain is the most basic of all R groups (a fact reflected in its pKa value of 12.5). As mentioned above for aspartate and glutamate, the side chains of arginine and lysine also form ionic bonds. The chemical structures of Group IV amino acids are
The imidazole side chain of histidine allows it to function in both acid and base catalysis near physiological pH values. None of the other standard amino acids possesses this important chemical property. Therefore, histidine is an amino acid that most often makes up the active sites of protein enzymes.
The majority of amino acids in Groups II, III, and IV are hydrophilic (“water loving”). As a result, they are often found clustered on the surface of globular proteins in aqueous solutions.
Anions are negatively charged ions, and cations are positively charged ions. Zwitterions are ions with both a negative and a positive charge. In this lesson, we'll look at some examples of zwitterions and learn about their function.
What is a Zwitterion?
Let's take the word zwitterion apart to figure out what it means. There's the -ion part at the end, which is a chemical species with a charge. The first part of the name comes from the German word 'zwitter', meaning hermaphrodite or hybrid. In other words, this term means half anion and half cation. Zwitterions are sometimes called dipolar ions, because they have a negative end (the anion) and a positive end (the cation).
Amino Acids as Zwitterions
Amino acids are the building blocks of proteins in living cells. They are compounds that contain an amino group and a carboxyl group. Twenty different amino acids are found in proteins. They share the structure shown here, where R represents one of the 20 possible side chains on an amino acid.
General structure of an amino acid
At neutral pH values, the amino group (-NH3+) has a positive charge and the carboxyl group (COO-) has a negative charge. Here is what the simplest amino acid, glycine, looks like in its zwitterion form:
Zwitterion form of glycine
The other interesting thing about zwitterions is if there are no other charged groups in the molecule, then a zwitterion has no net charge. That's right - a positive charge (+1) and a negative charge (-1) add up to zero.
Zwitterion Character Depends on pH
Zwitterions form of a compound at neutral pH, but it turns out that a zwitterion isn't always a zwitterion. At pH values far above or far below 7, its groups can take on different charges - or become neutral. At low, or acidic, pH values, the hydrogen ions add to the carboxyl group, making it neutral. This gives the amino acid a net charge of +1. At high, or basic, pH values, a hydrogen ion on N is removed by the excess base, neutralizing the amino group. This gives the amino acid a net charge of -1
No comments:
Post a Comment