Protein Structure and Function

Introduction

Proteins are essential macromolecules in all living organisms, responsible for a wide variety of biological functions. Their structure is intricately related to their function, and understanding this relationship is fundamental in biology.

Biological Roles of Proteins

Functions of Proteins in Cells

• Sensing light: Proteins such as opsins in the eye detect and respond to light stimuli.

• Defending cells against viruses: Antibodies and other immune proteins recognize and neutralize pathogens.

• Breaking down food polymers: Enzymes catalyze the hydrolysis of complex molecules into simpler ones for absorption.

• Changing cell shape: Structural proteins like actin and tubulin are involved in cell movement and maintaining cell shape.

• Summary: All of the above processes directly involve proteins.

The Flow of Genetic Information

Central Dogma of Molecular Biology

• DNA contains genetic instructions.

• Messenger RNA (mRNA) is transcribed from DNA and carries the code for protein synthesis.

• Proteins are synthesized based on the sequence of nucleotides in mRNA and determine physical traits.

Example: The color of an animal's fur is determined by proteins encoded by specific genes.

Proteins as Polymers

Monomers of Proteins

• Proteins are polymers of amino acids.

• Each protein is made by linking amino acids in a specific sequence.

Amino Acids: Structure and Properties

General Structure of Amino Acids

• All amino acids have a central carbon atom (the alpha carbon), bonded to:

  • An amino group ()

  • A carboxyl group ()

  • A hydrogen atom

  • A unique side chain (R group)

Key functional groups present on every amino acid: an amino group and a carboxyl group.

Classification of Amino Acids by R Group

• Nonpolar amino acids: R group contains only hydrogen and carbon atoms. These are hydrophobic and do not form hydrogen bonds.

• Polar amino acids: R group contains electronegative atoms (like O or N) and can form hydrogen bonds.

• Charged amino acids: R group is either positively (basic) or negatively (acidic) charged at physiological pH.

Example: If a newly discovered amino acid has an R group with only hydrogen and carbon, it is nonpolar.

Functional Groups and Hydrogen Bonding

• Amino (-NH2) and hydroxyl (-OH) groups can participate in hydrogen bonding.

• Other groups like sulfhydryl (-SH) can also form specific bonds (e.g., disulfide bonds).

Peptide Bond Formation

Condensation and Hydrolysis Reactions

• Condensation reaction: Joins two amino acids by forming a peptide bond and releasing water.

• Hydrolysis: Breaks a peptide bond by adding water, releasing individual amino acids.

Equation for peptide bond formation:

Levels of Protein Structure

Primary Structure

• The unique sequence of amino acids in a polypeptide chain.

• Written from the amino (N-) terminus to the carboxyl (C-) terminus.

Secondary Structure

• Regular, repeated patterns formed by hydrogen bonds between backbone atoms.

• Common types:

  • Alpha helix (α-helix): Coiled structure stabilized by hydrogen bonds.

  • Beta pleated sheet (β-sheet): Sheet-like arrangement stabilized by hydrogen bonds.

• Hydrogen bonds are the main force stabilizing secondary structure.

Tertiary Structure

• The overall 3D shape of a single polypeptide chain.

• Stabilized by:

  • Hydrogen bonds

  • Hydrophobic interactions

  • Disulfide bonds (covalent bonds between cysteine residues)

  • Van der Waals interactions

  • Ionic bonds

Quaternary Structure

• Association of two or more polypeptide chains (subunits) into a functional protein complex.

• Examples: Hemoglobin (tetramer), DNA-binding proteins (dimers).

Summary Table: Levels of Protein Structure

Protein Folding and Function

Importance of Folding

• Proper folding is essential for protein function.

• Denaturation (loss of structure) leads to loss of function.

• Some proteins require binding of ions or other molecules to achieve their active conformation (e.g., calmodulin binds Ca2+).

Enzyme Specificity

• Enzymes are proteins that catalyze specific chemical reactions.

• Active site: The region of the enzyme where substrate binding and catalysis occur.

• The specificity of an enzyme is determined by the geometry and amino acid composition of its active site.

Effects of Amino Acid Changes

Mutations and Protein Function

• Changing a single amino acid (point mutation) can alter the primary structure.

• This may or may not affect the tertiary structure and function, depending on the location and nature of the change.

• Example: Sickle cell anemia is caused by a single amino acid substitution in hemoglobin, altering its function and shape.

Summary Table: Amino Acid Properties

Additional info: These notes expand on the provided slides by including definitions, examples, and summary tables for clarity and completeness.