CHE110 Chapter 21 PPT UPDATED

Chapter 21: Proteins - Lecture Outline

Introduction to Proteins

• Definition: Proteins are complex biomolecules composed of amino acids joined by peptide bonds (amide bonds).

• Composition: They account for approximately 50% of the dry weight of the human body.

• Dietary Requirement: Unlike carbohydrates and lipids, proteins are not stored in the body; thus, a daily intake of proteins is necessary.

  • Recommended dietary intake for adults: 0.8 grams of protein per kg of body weight (children require more).

  • Major dietary sources of protein: Meat and milk.

Amino Acids

General Features of Amino Acids

• Structure: Amino acids consist of an amino group (-NH2) and a carboxyl group (-COOH).

  • The simplest amino acid is glycine (R = H).

• R Group (Side Chain): Determines the identity of the amino acid.

  • Basic amino acid: R contains a basic nitrogen atom.

  • Acidic amino acid: R contains an additional -COOH group.

  • Neutral amino acids: All other structures not falling into the above categories.

• Zwitterion Formation: Amino acids can exist as zwitterions, where a proton transfers between the acid and the amino group, resulting in a neutral molecule that has both a positive and negative charge.

Common Amino Acids

• 20 Common Naturally Occurring Amino Acids: Listed with characteristics and structures.

• Amino Acid Classification:

  • Neutral Amino Acids: 15 examples including Alanine, Glycine, Isoleucine, etc.

  • Acidic Amino Acids: Aspartic acid and Glutamic acid.

  • Basic Amino Acids: Arginine, Histidine, and Lysine.

• Essential Amino Acids: Include Isoleucine, Leucine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine, Arginine, Histidine, and Lysine.

Acid-Base Behavior of Amino Acids

• Isoelectric Point: The pH at which the amino acid exists as a zwitterion.

• Charge Variation with pH:

  • Below isoelectric pH: The amino acid has a net positive charge (protonation of carboxyl group).

  • Above isoelectric pH: The amino acid has a net negative charge (deprotonation of amino group).

Peptides and Proteins

Formation of Peptides

• Peptide Bonds: Amino acids join through amide bonds to form dipeptides, tripeptides, and polypeptides.

  • Dipeptide: Two amino acids joined by one peptide bond.

  • Tripeptide: Three amino acids joined by two peptide bonds.

  • Polypeptide: Many amino acids; Proteins consist of more than 40 amino acids.

Drawing Dipeptides

• Instructions on drawing a dipeptide from two amino acids, using Valine and Glycine as examples.

  • N-terminal amino acid is written on the left; C-terminal amino acid on the right.

  • Example provided for dipeptide formation: Val-Gly.

Focus on the Human Body

• Examples of Proteins:

  • Hemoglobin: Transports oxygen in blood.

  • Collagen: Structural protein in connective tissue.

  • Ferritin: Iron storage protein.

  • Actin & Myosin: Involved in muscle contraction.

  • Insulin: Hormonal protein regulating blood sugar.

Advanced Protein Structure and Function

Levels of Protein Structure

• Primary Structure: Sequence of amino acids.

• Secondary Structure: Includes patterns like alpha helices and beta sheets formed through hydrogen bonding.

• Tertiary Structure: The 3D shape of a polypeptide due to interactions such as disulfide bonds, hydrogen bonds, and others.

• Quaternary Structure: Assembled from multiple polypeptides.

Examples of Protein Structures

• Hydrogen Bonds: Stabilize protein structure in alpha helices and beta sheets.

• Disulfide Bonds: Can form intramolecular or intermolecular connections.

  • Example: Linkages in insulin between specific residues.

Enzyme Reactions

• Enzymatic Action: Enzymes catalyze biochemical reactions; for example, converting lactate to pyruvate involves lactate dehydrogenase.

• Importance of Active Site: Enzymes have specific active sites where substrates bind, forming enzyme-substrate complexes.

  • Different types of inhibitors can affect enzyme activity.

Conclusion

• Proteins are vital to many biological functions, playing roles in structure, regulation, and catalysis within living organisms.