Amino Acids & Proteins

Introduction to Amino Acids

Amino acids are the building blocks of proteins, each consisting of a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R-group). The properties of the R-group determine the chemical nature and function of each amino acid.

• Definition: An amino acid is an organic molecule containing both an amino group (-NH2) and a carboxyl group (-COOH).

• Classification: Amino acids are classified based on the properties of their R-groups:

  • Non-polar: R-groups are mostly hydrocarbons; hydrophobic in nature.

  • Polar (uncharged): R-groups contain oxygen or nitrogen, creating dipoles but not charges.

  • Acidic (negatively charged): R-groups have lost a hydrogen from a carboxylic acid group (-COO-).

  • Basic (positively charged): R-groups have gained a hydrogen on an amine group (-NH3+).

• Example: Glutamic acid is acidic; Lysine is basic; Serine is polar; Leucine is non-polar.

Structural Features of Amino Acids

The structure of an amino acid determines its classification and its role in protein structure and function.

• Key structural features:

  • Central (alpha) carbon

  • Amino group (-NH2)

  • Carboxyl group (-COOH)

  • Hydrogen atom

  • R-group (side chain)

• R-group properties: Influence solubility, reactivity, and protein folding.

Protein Structure: Levels of Organization

Primary Structure

The primary structure of a protein is its linear sequence of amino acids joined by peptide bonds. This sequence determines all higher levels of protein structure.

• Peptide bond: Covalent bond formed between the carboxyl group of one amino acid and the amino group of another.

• Flexibility: Single bonds in the backbone allow rotation, but the peptide bond itself is planar and rigid.

• Notation: Sequence is written from N-terminus (amino end) to C-terminus (carboxyl end).

• Example: 0

Secondary Structure

Secondary structure refers to local folding patterns stabilized by hydrogen bonds between backbone atoms. The most common types are alpha-helices and beta-sheets.

• Alpha-helix: Right-handed coil stabilized by hydrogen bonds every fourth amino acid.

• Beta-sheet: Extended strands connected laterally by hydrogen bonds, forming sheet-like structures.

• Beta-turn: Short regions that reverse the direction of the polypeptide chain.

• Stabilization: Hydrogen bonds between backbone atoms (not side chains).

• Example: Hemoglobin contains both alpha-helices and beta-sheets.

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a single polypeptide chain, determined by interactions among side chains (R-groups) and between side chains and the backbone.

• Stabilizing interactions:

  • Hydrogen bonds

  • Ionic bonds

  • Van der Waals interactions

  • Disulfide bonds (covalent bonds between cysteine residues)

  • Hydrophobic interactions

• Example: Disulfide bonds in insulin stabilize its tertiary structure.

Quaternary Structure

Quaternary structure describes the arrangement of multiple polypeptide subunits in a protein complex. Stabilization is similar to tertiary structure, involving non-covalent interactions and sometimes covalent bonds.

• Example: Hemoglobin consists of four polypeptide subunits (two alpha and two beta chains).

Classification of Amino Acids by R-group Properties

Summary Table: Amino Acid Categories

The following table summarizes the main categories of amino acids based on their R-group properties:

Impact of Amino Acid Sequence on Protein Structure and Function

Sequence Determines Structure and Function

The specific order of amino acids in a protein (primary structure) dictates how the protein folds and its final three-dimensional shape, which in turn determines its biological function.

• Mutations: A change in the amino acid sequence can alter protein folding, stability, and activity.

• Example: Sickle cell anemia is caused by a single amino acid substitution in hemoglobin.

Summary: Levels of Protein Organization

Practice and Application

Identifying Amino Acid Types

To classify an amino acid, examine its R-group:

• If the R-group is mostly carbon and hydrogen, it is likely non-polar.

• If the R-group contains oxygen or nitrogen, it is likely polar (uncharged).

• If the R-group has a carboxylate group (-COO-), it is acidic.

• If the R-group has an amino group (-NH3+), it is basic.

Application: Use online tools such as PepDraw to visualize and analyze peptide sequences.

Additional info: The notes reference the citric acid cycle and enzymes, which are related topics in protein biochemistry and metabolism, but the main focus here is on amino acids and protein structure.