Amino Acids Overview

Amino Acids

• The building blocks of proteins
• Only 20 amino acids are commonly found in natural proteins
• Alpha-amino acid: Contains both a carboxyl and amino functional group
• The amino group is directly attached to the carbon next to the carboxyl group
• Each amino acid has a name, a three-letter abbreviation, and a one-letter abbreviation.

Amino Acids and R Groups

• Amino acids are categorized into groups based on the polarity of the R groups
• Neutral and nonpolar side chains are hydrophobic
• Polar side chains are hydrophilic

Amino Acids - Ionizable Groups

• Amino acids exist in solution as zwitterions
• A zwitterion is a molecule that is electrically neutral, but contains both a positive and a negative charge
• In acidic solutions, the carboxylate group accepts a proton, and the zwitterion becomes positively charged
• In basic solutions, the amino group loses a proton, and the zwitterion becomes negatively charged

Reaction of Amino Acids - Oxidation

• Cysteine, the only sulfhydryl (—SH)-containing amino acid, can easily be oxidized to form a disulfide bond (—S—S—)

Reactions of Amino Acids - Peptide Formation

• Peptide bond formation enables amino acids to make polymers by forming amide linkages
• Dipeptides are compounds formed when two amino acids are bonded by a peptide bond
• A peptide bond or peptide linkage is an amide linkage between amino acids that results when the amino group of one acid reacts with the carboxylate group of another
• The order of linkage is important because two different dipeptides can be formed when two unique amino acids react
• Peptide: Amino acid polymer of short chain length
• Polypeptide: Intermediate chain length polymer with less than 50 amino acids
• Protein: Polymer with more than 50 amino acids
• Amino acid residue: Amino acid that is a part of a peptide, polypeptide, or protein chain

Classification of Proteins Based on Composition

• Simple proteins: Made up entirely of amino acid residues
• Conjugated proteins: Contain amino acid residues and other organic or inorganic components
• Prosthetic groups: Non–amino acid parts of conjugated proteins

Primary Protein Structure

• Linear sequence of amino acid residues in a protein chain
• Determines secondary and tertiary structures
• Important for the functioning of proteins
• Small variations in the primary structure can cause profound differences in the functioning of proteins

Secondary Protein Structure

• Arrangement of protein chains into patterns as a result of hydrogen bonds between amide groups of amino acid residues in the chain
• Includes:
  • α-helix
  • β-pleated sheet

α-Helix Structure

• Occurs when a single protein chain twists to resemble a coiled helical spring
• Held in shape by intramolecular hydrogen bonds between carbonyl oxygens and amide hydrogens in adjacent turns of the helical backbone
• All amide groups in the helix are hydrogen bonded
• Two or more helices interact to form a cable in α-Keratin (found in hair), myosin (found in muscle), epidermin (found in skin), and fibrin (found in blood clots)

β-Pleated Sheet

• Occurs when several protein chains lie side by side and are held in position by hydrogen bonds between the carbonyl oxygen atoms of one chain and the amide hydrogen atoms of an adjacent chain
• Found extensively only in silk protein

Tertiary Protein Structure

• Specific three-dimensional shape of a protein resulting from interactions between R groups of amino acid residues
• Types of R-group interactions:
  • Disulfide bridges: Form between cysteine residues
  • Salt bridges: Result of ionic bonds that form between the ionized side chain of an acidic amino acid and the side chain of a basic amino acid
  • Hydrogen bonds: Form between side chains that possess the following functional groups: —OH, —SH, —NH2, and —C=O
  • Hydrophobic interactions: Form between nonpolar residues
• The interaction of hydrophilic and hydrophobic side chains with water determines shape in an aqueous environment.

This quiz covers the essential concepts of amino acids, including their structure, categorization based on R groups, and their behavior in different pH solutions. Understand the fundamental role of amino acids as the building blocks of proteins and how their properties influence biological function.