Amino Acids: Building Blocks of Proteins
Amino Acids: The Monomers of Proteins
Amino acids are the building blocks of peptides and proteins. Over twenty amino acids are found in proteins and are common to all living organisms.
Molecular Structure
All protein amino acids share a common structure: an amino group (-NH2), which is basic, and a carboxyl group (-COOH), which is acidic. They also have a unique side chain (R group) that varies among different amino acids.
Classification of Amino Acids
Amino acids are classified into four groups based on their side chains:
- Neutral Polar Amino Acids: At pH 7, these side chains are hydrophobic, decreasing solubility.
- Polar Neutral Amino Acids: At pH 7, these side chains are hydrophilic due to uncharged polar groups (-OH, -NH2, -SH), allowing them to form hydrogen bonds with water.
- Acidic Amino Acids: Negatively charged due to a side-chain carboxyl group, donating electrons at low pH.
- Basic Amino Acids: Positively charged due to a side-chain amino group, capturing protons as pH rises.
Essential Amino Acids
Heterotrophic organisms can synthesize some amino acids from simpler compounds, but others, called essential amino acids, must be obtained from the diet. Autotrophic organisms can synthesize all amino acids.
Properties of Amino Acids
- Simple Molecular Structure: Organic compounds, water-soluble, crystallizable, colorless, with high melting points, making them solid.
- Stereoisomerism: All amino acids except glycine have an asymmetric carbon (Cα), leading to stereoisomerism. Each amino acid can have two stereoisomers (D and L configurations). Protein amino acids have the L configuration.
- Amphoteric Behavior: Amino acids can act as acids or bases depending on the pH of the solution. In neutral solutions, they exist as zwitterions (both positive and negative charges).
Isoelectric Point (pI)
The pH at which an amino acid has no net charge is called the isoelectric point (pI). The amphoteric nature of amino acids allows them to act as buffers, maintaining pH stability in biological systems.
Protein Formation and Structure
Peptide Bond
Amino acids are linked by peptide bonds to form peptides and proteins. This bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing a water molecule. Peptide bonds can be broken by hydrolysis.
Peptide Bond Characteristics
- Covalent amide bond.
- Partial double bond character, making it rigid and preventing rotation.
- Cα bonds can rotate.
- Usually in trans configuration.
Peptides
- Oligopeptides: Contain 2-10 amino acids.
- Polypeptides: Contain more than 10 amino acids.
Protein Structure
Proteins have four levels of structure:
- Primary Structure: The linear sequence of amino acids.
- Secondary Structure: Local folding patterns (α-helix and β-sheet) stabilized by hydrogen bonds.
- Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain.
- Quaternary Structure: The arrangement of multiple polypeptide chains (subunits) in a protein complex.