Lecture Notes: Amino Acids, Peptide Bonds, and Proteins

Proteins: Structure and Function

Introduction to Proteins

  • Proteins are the most structurally complex and functionally sophisticated molecules known.

  • The structure and chemistry of each protein have been fined-tuned over billions of years of evolutionary history.

  • The location of each amino acid in the long string of amino acids that forms a protein determines its three-dimensional shape.

  • The precise shape of each protein molecule determines its function in a cell.

Amino Acids and Their Role in Protein Structure

  • Amino Acid Structure: The general formula of an amino acid includes:

    • alpha-carbon atom (Cα)

    • Amino group (NH₂)

    • Carboxyl group (COOH)

    • Side-chain group (R), which is one of 20 different side chains (e.g., alanine is Ala or A).

  • At pH 7, both the amino and carboxyl groups are ionized (undergo protonation):

    • Amino group: +H₃N-C-COO

  • Optical Isomers: The alpha-carbon atom can form two mirror images or stereo-isomers known as L (levo) and D (dextro). Proteins consist exclusively of L-amino acids.

Families of Amino Acids

  • Amino acids are classified according to the properties of their side chains:

    • Acidic: Aspartic acid (Asp, D), Glutamic acid (Glu, E)

    • Basic: Lysine (Lys, K), Arginine (Arg, R), Histidine (His, H)

    • Uncharged Polar: Asparagine (Asn, N), Glutamine (Gln, Q), Serine (Ser, S), Threonine (Thr, T), Tyrosine (Tyr, Y)

    • Nonpolar: Alanine (Ala, A), Valine (Val, V), Methionine (Met, M), Tryptophan (Trp, W), Leucine (Leu, L), Isoleucine (Ile, I), Glycine (Gly, G), Proline (Pro, P), Phenylalanine (Phe, F), Cysteine (Cys, C)

  • Each amino acid has a single-letter abbreviation and a three-letter code.

Key Features of Amino Acids

  • Acidic Side Chains:

    • Aspartic acid (Asp, or D)

    • Glutamic acid (Glu, or E)

  • Basic Side Chains:

    • Lysine (Lys, K) with a positive charge stabilized by resonance.

    • Arginine (Arg, R)

    • Histidine (His, H) with an imidazole group (pKa ~6.0) providing buffered power near neutral pH.

  • Nonpolar Side Chains:

    • Examples include Alanine (Ala, or A), Glycine (Gly, or G), Valine (Val, or V), etc.

  • Uncharged Polar Side Chains:

    • Asparagine (Asn, N), Glutamine (Gln, Q), Serine (Ser, S), Threonine (Thr, T), and Tyrosine (Tyr, Y), which have polar groups.

Peptide Bonds and Protein Structure

  • Peptide Bonds: Amino acids are commonly joined through an amide linkage called a peptide bond, resulting in the formation of long polymers which are proteins. Peptide bonds are rigid planar units due to restricted rotation around the C-N bond.

  • Proteins are written with the N-terminus (amino) on the left and the C-terminus (carboxyl) on the right.

  • Characteristics of Peptide Bonds: The rigidity of the peptide bond limits the flexibility of the polypeptide chain. Only the two single bonds attached allow for rotation, leading to diverse structural configurations in proteins.

Protein Structure Levels

  • Primary Structure: Sequential arrangement of amino acids in a polypeptide chain.

  • Secondary Structure: Local folding patterns (e.g., alpha-helix and beta-sheet) stabilized mainly by hydrogen bonds in the polypeptide backbone.

    • Alpha helix: A right-handed spiral structure stabilized by hydrogen bonds.

    • Beta sheet: Arranged in parallel or anti-parallel form, also stabilized by hydrogen bonds.

  • Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain, formed by interactions between side chains (R groups).

  • Quaternary Structure: Involves multiple polypeptide chains (subunits) that come together to form a functional protein.

Protein Folding and Stability

  • The shape and structure of proteins are influenced by hydrogen bonding, van der Waals forces, electrostatic interactions, and hydrophobic effects.

  • The crowded cellular environment promotes folding of polypeptides, leading to unique configurations necessary for their biological functions.

Post-Translational Modifications

  • Proteins can undergo various modifications after translation, affecting their function and structure, such as phosphorylation, methylation, acetylation, etc.

  • Example: Phosphoserine, Phosphothreonine, Phosphotyrosine are all modified amino acids commonly seen in proteins influencing signaling and cellular responses.

Protein-Protein Interaction and Binding

  • All proteins bind to other molecules known as ligands, typically through non-covalent interactions (hydrogen bonds, ionic bonds, and hydrophobic interactions).

  • The specific area where the ligand binds on the protein is called the binding site, often facilitated by the conformation of the protein surface.

  • The arrangement of amino acids in the binding site is critical for protein function and specificity.

Conclusion

  • Understanding the structure and various interactions of amino acids and proteins is crucial in biochemistry and molecular biology, providing insights into protein function and the intricate mechanisms underlying all biological processes.