Cellular Foundations of Life

Introduction to Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life, including carbohydrates, proteins, lipids, and nucleic acids. Their structure and function are determined by the arrangement of their monomers and the types of bonds formed during polymerization.

• Monomers: Small, repeating units that join to form polymers.

• Polymers: Large molecules composed of monomers linked by covalent bonds.

• Dehydration Reaction: A chemical reaction that joins monomers by removing water, forming specific linkages.

Table: Macromolecule Monomers, Polymers, and Linkages

Proteins: Structure and Function

Overview of Protein Functions

Proteins are versatile macromolecules that perform a wide range of functions in living organisms. Their function is determined by their structure, which is in turn dictated by the sequence and properties of their amino acids.

• Enzymatic proteins: Accelerate chemical reactions (e.g., digestive enzymes).

• Defensive proteins: Protect against disease (e.g., antibodies).

• Storage proteins: Store amino acids (e.g., casein in milk, ovalbumin in egg white).

• Transport proteins: Move substances (e.g., hemoglobin transports oxygen).

• Hormonal proteins: Coordinate activities (e.g., insulin regulates blood sugar).

• Receptor proteins: Respond to chemical stimuli (e.g., nerve cell receptors).

• Contractile and motor proteins: Enable movement (e.g., actin and myosin in muscles).

• Structural proteins: Provide support (e.g., keratin in hair, collagen in connective tissue).

Structure of Amino Acids

Amino acids are the building blocks of proteins. Each amino acid has a central carbon atom (alpha carbon) bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).

• Alpha carbon: Central atom in the amino acid structure.

• Amino group:

• Carboxyl group:

• R group/side chain: Determines the properties and identity of the amino acid.

Forms of Amino Acids

• Non-ionized form:

• Ionized form:

Classification of Amino Acids by R Group Properties

• Nonpolar (hydrophobic): e.g., Alanine, Valine, Leucine, Phenylalanine, Tryptophan

• Polar (hydrophilic): e.g., Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine

• Charged (hydrophilic):

  • Acidic (negatively charged): Glutamic acid

  • Basic (positively charged): Lysine, Arginine

Protein Structure: Levels of Organization

Primary Structure

The primary structure of a protein is its unique sequence of amino acids, determined by genetic information. This sequence dictates all higher levels of structure and ultimately the protein's function.

• Peptide bonds link amino acids in a linear chain.

• Sequence is written from the amino (N) terminus to the carboxyl (C) terminus.

Secondary Structure

Secondary structure refers to local folding patterns stabilized by hydrogen bonds between backbone atoms. The two main types are:

• Alpha helix (-helix): Coil structure stabilized by hydrogen bonds every fourth amino acid.

• Beta pleated sheet (-sheet): Sheet-like structure formed by hydrogen bonds between segments of the polypeptide chain.

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a polypeptide, resulting from interactions between R groups (side chains), including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

• Hydrophobic interactions: Nonpolar side chains cluster away from water.

• Disulfide bridges: Covalent bonds between cysteine residues.

• Van der Waals forces: Weak attractions between molecules.

Quaternary Structure

Quaternary structure arises when two or more polypeptide chains (subunits) associate to form a functional protein complex. Examples include hemoglobin and transthyretin.

• Subunit interactions are crucial for function.

• Can affect properties such as stability and activity.

Protein Shape and Denaturation

Importance of Protein Structure

The function of a protein is directly related to its shape. Changes in temperature, pH, or salt concentration can disrupt the bonds maintaining protein structure, leading to denaturation and loss of function.

• Denaturation: Loss of native structure and biological activity.

• Renaturation is sometimes possible if normal conditions are restored.

Types of Proteins: Globular vs. Fibrous

Comparison Table

Summary

Proteins are essential macromolecules with diverse functions, determined by their structure at four hierarchical levels. Understanding the relationship between amino acid sequence, structure, and function is fundamental to biology and medicine.

Example: Hemoglobin is a globular protein with quaternary structure, responsible for oxygen transport in blood. Collagen is a fibrous protein providing structural support in connective tissues.

Additional info: The notes cover foundational concepts in protein chemistry, suitable for introductory college biology.