NESA Chapter 18 (Amino Acids and Proteins) (1)

Chapter 2 Lecture: Amino Acids and Proteins

Fundamentals of General, Organic, and Biological Chemistry 8th Edition

• Source: Lecture by Christina A. Johnson, Ph.D. at the University of California, San Diego.

18.1 An Introduction to Biochemistry

• Biochemistry: Study of molecules and their reactions in living organisms.

  • Builds on principles from prior chapters (1-17).

  • Nutritionists use biochemistry to evaluate dietary needs.

  • The pharmaceutical industry employs biochemistry to design molecules that mimic or alter biomolecule actions.

• Goal: Understand structures of biomolecules and their structure-function relationships.

• Common Ground: Biochemistry connects various life sciences, answering fundamental questions at the molecular level.

• Principal Classes of Biomolecules:

  • Proteins

  • Carbohydrates

  • Lipids

  • Nucleic acids

• Function of Biochemical Reactions:

  • Break down food

  • Generate and store energy

  • Build new biomolecules

  • Eliminate waste

• Complexity of Biomolecules: Despite their size, they share functional groups and chemical reactions like simpler organic molecules.

18.2 Proteins and Their Functions: An Overview

• Composition: Approximately 50% of the body’s dry weight is protein.

• Etymology: "Protein" originates from the Greek word "proteios," meaning "primary."

• Roles of Proteins:

  • Provide structure (e.g., keratin, actin filaments) and support to tissues and organs.

  • Act as hormones (e.g., oxytocin) and enzymes (e.g., catalase) that control metabolism.

  • Pick up and transport molecules in body fluids (e.g., casein, transferrin).

  • Protect against invaders (e.g., Immunoglobulin G).

Classification of Proteins by Function (Table 18.2)

• Types of Proteins:

  • Enzymes: Catalyze reactions (e.g., Amylase).

  • Hormones: Integral in body functions (e.g., Insulin).

  • Storage Proteins: Store substances (e.g., Myoglobin).

  • Transport Proteins: Carry substances (e.g., Serum albumin).

  • Structural Proteins: Provide support (e.g., Collagen).

  • Protective Proteins: Defend against pathogens (e.g., Immunoglobulin).

  • Contractile Proteins: Facilitate movement (e.g., Myosin, Actin).

18.3 Amino Acids

• Definition: Proteins are polymers of amino acids.

• Structure of Amino Acids:

  • Each contains an amino group (–NH2), a carboxyl group (–COOH), and a variable R group/side chain.

  • Alpha Carbon: Central carbon to which these groups attach.

• Distinct R Groups:

  • Differentiate amino acids (may be hydrocarbons or contain functional groups).

• Total Count: 20 standard amino acids contribute to protein formation, each with a unique three-letter abbreviation.

• Proline Exception: Considered a secondary amine and has a distinct structure.

• Classification of Amino Acids:

  • Neutral, acidic, or basic dependent on the nature of side chains.

  • Neutral divided into polar and nonpolar groups.

Intermolecular Forces in Amino Acids

• Importance of Intermolecular Forces:

  • Includes hydrogen bonds, Van der Waals forces, ionic bonds, and disulfide bridges.

  • Nonpolar side chains are hydrophobic (avoid water), while polar side chains are hydrophilic (attract water).

18.4 Acid-Base Properties of Amino Acids

• Zwitterions: Neutral dipolar ions with one positive and one negative charge.

• Acidic Group: (form a zwitterion by intramolecular interactions).

• Behavior in Solutions:

  • Acidic Solutions: Zwitterions accept protons, leading to a predominant positive state.

  • Basic Solutions: They lose protons, leading to a predominant negative state.

18.5 Peptides

• Structure Formation:

  • Peptide bond: Amide bond connecting two amino acids via –NH2 and –COOH groups.

  • Dipeptide: Two amino acids.

  • Tripeptide: Three amino acids.

  • Any linked amino acids form a polypeptide; extensive chains are referred to as proteins.

• Importance of Sequence: Sequence influences protein identity and function.

18.6 Primary Protein Structure (1°)

• Definition: Sequence of amino acids in a protein chain.

• Backbone Composition: Alternates between peptide bonds and alpha-carbon atoms.

• Stability and Function: Correct sequence is crucial; even single amino acid changes can affect biological properties.

18.7 Secondary Protein Structure (2°)

• Types:

  • Alpha-helix: Right-handed coil stabilized by hydrogen bonding.

  • Beta-sheet: Flat structure formed by hydrogen bonds between adjacent chains.

• Two Classifications: Fibrous (e.g., keratin) vs. globular proteins (e.g., enzymes).

18.8 Tertiary Protein Structure (3°)

• Definition: Overall three-dimensional shape of a protein determined by various forces.

• Key Interactions: Include hydrogen bonds, ionic attractions (salt bridges), hydrophilic and hydrophobic interactions, and disulfide bridges.

• Conjugated Proteins: Contain non-amino acid components that aid function (e.g., myoglobin).

18.9 Quaternary Protein Structure (4°)

• Definition: Aggregation of two or more protein chains into larger structures.

• Example—Hemoglobin: Composed of four polypeptide chains, it carries oxygen and operates through specific interactions including hydrophobic interactions.

18.10 Chemical Properties of Proteins

• Hydrolysis: Peptide bonds are broken yielding amino acids during digestion or chemical processes.

• Denaturation: Disruption of secondary and higher structures, impacting protein functionality.

• Agents Causing Denaturation: Include heat, mechanical agitation, detergents, organic solvents, pH changes, and high ionic concentrations.