21.7
Amino acids
Amino acids are the monomers that comprise proteins. Each amino acid has the same fundamental structure, which consists of a central carbon atom, or t…
Amino acids are organic compounds that are predominantly made up of carbon, hydrogen, oxygen, and nitrogen. Although there are hundreds of amino acids, there are twenty that function as building blocks of proteins. These twenty amino acids are often referred to by their three or one-letter abbreviations.
All amino acids have a central carbon called the alpha carbon, which functions as a chiral center for most amino acids. It is attached to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.
A chiral molecule has four distinct groups arranged around a particular atom, known as the chiral center. As glycine has a hydrogen atom as its R group, it does not have four distinct groups and therefore does not have a chiral center.
These four groups can form two distinct configurations that are stereoisomers, called L and D isomers. These isomers are mirror images that are not superimposable on each other. Amino acids used in protein synthesis are always L-isomers.
The amine group on an amino acid can act as a base and accept a proton resulting in a positive charge, whereas the carboxyl group can act as an acid and donate a proton resulting in a negative charge. At physiological pH, amino acids usually exist as dual-charged and are called zwitterions, molecules with an equal number of positive and negative charges.
Every amino acid has a unique R group that is responsible for their varied features that include their size, solubility, and charge. Amino acids are broadly classified into two categories based on their R-groups: non-polar and polar.
Non-polar amino acids can be divided into aliphatic and aromatic. Polar amino acids can be divided into acidic, basic, and uncharged at a neutral pH.
Aliphatic groups are single bonded hydrocarbons that exist as straight, branched, or cyclic chains. The amino acids with aliphatic R groups are glycine, the simplest amino acid; alanine, valine, leucine, and isoleucine, which are extremely hydrophobic and associate with each other to form stable core structures in proteins; methionine, a sulfur-containing amino acid; and proline, which has an aliphatic ring.
Aromatic groups are hydrocarbons that exist as unsaturated rings with alternating single and double bonds. The aromatic amino acids are phenylalanine, tyrosine, and tryptophan, all of which are large and capable of participating in hydrophobic interactions.
The acidic amino acids are aspartate and glutamate. They have carboxyl groups that donate a proton to become negatively charged. The basic amino acids are lysine, arginine, and histidine. They have amino groups on their side chains that accept a proton to become positively charged.
The polar uncharged amino acids are serine, threonine, cysteine, asparagine, and glutamine. They are hydrophilic and contain various polar functional groups: hydroxyl groups in serine and threonine, a sulfhydryl group in cysteine, and amide groups in asparagine and glutamine.
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Q1: What are the four groups attached to the alpha carbon in amino acids?
The alpha carbon in amino acids is bonded to four distinct groups: an amino group, a carboxyl group, a hydrogen atom, and a variable R group. These four groups arrange around the alpha carbon, which functions as a chiral center for most amino acids. The R group differs for each amino acid, determining its unique chemical properties and characteristics.
Q2: Why is glycine considered an exception among amino acids?
Glycine is the simplest amino acid because its R group is just a hydrogen atom, not a more complex side chain. Since it has only three distinct groups attached to its alpha carbon instead of four, glycine lacks a chiral center. This makes glycine unique among the twenty common amino acids used in protein synthesis.
Q3: What is a zwitterion and how do amino acids form them?
A zwitterion is a molecule with an equal number of positive and negative charges. At physiological pH, amino acids exist as zwitterions because the amino group accepts a proton and becomes positively charged, while the carboxyl group donates a proton and becomes negatively charged. This dual-charged state is the typical form of amino acids in living cells.
Q4: How do nonpolar and polar amino acids differ in their R groups?
Nonpolar amino acids have hydrophobic R groups that are either aliphatic or aromatic hydrocarbons. Polar amino acids have hydrophilic R groups containing functional groups like hydroxyl, sulfhydryl, or amide groups. The chemical nature of the R group determines whether an amino acid is acidic, basic, polar, or nonpolar, directly affecting its behavior in proteins.
Q5: What makes aromatic amino acids distinct from aliphatic amino acids?
Aromatic amino acids like phenylalanine, tyrosine, and tryptophan have R groups containing unsaturated rings with alternating single and double bonds. Aliphatic amino acids like glycine, alanine, and valine have single-bonded hydrocarbon R groups arranged as straight, branched, or cyclic chains. Both types are nonpolar and hydrophobic, but aromatic amino acids are larger and participate in hydrophobic interactions.
Q6: How do acidic and basic amino acids differ in their charge properties?
Acidic amino acids like aspartate and glutamate have carboxyl groups on their R groups that donate protons, becoming negatively charged. Basic amino acids like lysine, arginine, and histidine have amino groups on their side chains that accept protons, becoming positively charged. These charged R groups enable amino acids to participate in ionic interactions and affect protein structure and function.
Q7: Why are L-isomers the only amino acids used in protein synthesis?
Amino acids exist as L and D isomers, which are stereoisomers and mirror images that cannot be superimposed on each other. Proteins exclusively use L-isomers because biological systems evolved to recognize and incorporate only this configuration during protein synthesis. This specificity ensures consistent protein structure and function across all living organisms.