Amino Acids and the Primary Structure of Proteins

Amino Acids

Introduction to Amino Acids

Amino acids are the fundamental building blocks of proteins, which are essential macromolecules in all living organisms. Each amino acid contains a central carbon atom (the α-carbon) bonded to an amino group, a carboxylic acid group, a hydrogen atom, and a unique side chain (R group) that determines its properties.

• α-Carbon: The central carbon atom to which all functional groups are attached.

• Amino Group (-NH2): Acts as a base and can accept a proton.

• Carboxyl Group (-COOH): Acts as an acid and can donate a proton.

• Side Chain (R group): Unique to each amino acid; determines the amino acid's identity and properties.

At neutral pH (~7), the amino group is typically protonated (-NH3+) and the carboxyl group is deprotonated (-COO-), resulting in a zwitterion (a molecule with both positive and negative charges but overall neutral).

Structure of α-Amino Acids

Chemical Structure and Zwitterions

The general structure of an α-amino acid can be represented as:

• General formula: $\mathrm{H_2N{-}CHR{-}COOH}$

• At physiological pH, the structure is: $\mathrm{^+H_3N{-}CHR{-}COO^-}$

• Zwitterion: A molecule with both a positive and a negative charge, but overall electrically neutral.

Example: Glycine is the simplest amino acid, with R = H.

Amino Acid Stereochemistry

Chirality and Stereoisomers

Most α-amino acids (except glycine) have four different groups attached to the α-carbon, making it a chiral center (asymmetric carbon). This gives rise to two possible stereoisomers, known as enantiomers:

• L-amino acids: The form found in proteins in nature.

• D-amino acids: The mirror image of L-amino acids; rarely found in proteins.

• Enantiomers: Non-superimposable mirror images of each other.

Example: L-alanine and D-alanine are enantiomers.

Classification of Naturally Occurring Amino Acids

The 20 Common Amino Acids

Proteins are composed of 20 standard amino acids, each with a unique side chain. These amino acids can be classified based on the properties of their side chains:

• Nonpolar, Aliphatic: Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Methionine

• Aromatic: Phenylalanine, Tyrosine, Tryptophan

• Polar, Uncharged: Serine, Threonine, Cysteine, Asparagine, Glutamine

• Positively Charged (Basic): Lysine, Arginine, Histidine

• Negatively Charged (Acidic): Aspartate, Glutamate

Each amino acid is represented by a three-letter and a one-letter code (e.g., Alanine: Ala, A).

Table: Classification of the 20 Common Amino Acids

Class	Amino Acids	Side Chain Properties
Nonpolar, Aliphatic	Glycine (Gly, G), Alanine (Ala, A), Valine (Val, V), Leucine (Leu, L), Isoleucine (Ile, I), Proline (Pro, P), Methionine (Met, M)	Hydrophobic, do not interact favorably with water
Aromatic	Phenylalanine (Phe, F), Tyrosine (Tyr, Y), Tryptophan (Trp, W)	Contain aromatic rings, absorb UV light
Polar, Uncharged	Serine (Ser, S), Threonine (Thr, T), Cysteine (Cys, C), Asparagine (Asn, N), Glutamine (Gln, Q)	Form hydrogen bonds, more hydrophilic
Positively Charged (Basic)	Lysine (Lys, K), Arginine (Arg, R), Histidine (His, H)	Basic side chains, positively charged at physiological pH
Negatively Charged (Acidic)	Aspartate (Asp, D), Glutamate (Glu, E)	Acidic side chains, negatively charged at physiological pH

Additional info: The classification helps predict the behavior of amino acids in proteins, such as their location (interior or surface) and their role in protein function.