Tertiary Structure of Proteins

Tertiary Structure

• The tertiary structure of a protein is the three-dimensional shape formed by the further folding of alpha helices and beta sheets.
• Tertiary structure is stabilized through interactions between the R groups of amino acids.
• These interactions include hydrogen bonds, ionic interactions, hydrophobic interactions, disulfide bonds, and Van der Waals forces.
• Hydrogen bonds form between polar R groups.
• Ionic interactions occur between oppositely charged R groups, attracting or repelling each other.
• Hydrophobic interactions occur between nonpolar R groups, causing them to cluster together.
• Disulfide bonds are the strongest interaction, formed between the sulfur-containing R groups of two cysteine amino acids.
• Van der Waals interactions are weak interactions between nonpolar R groups.
• The final folded structure of a protein is called its native conformation, which is biologically active.
• Proteins composed of a single polypeptide chain have their tertiary structure as their final active form.

Quaternary Structure

• Quaternary structure arises when multiple polypeptide chains (subunits) interact to form a protein.
• The same types of bonds involved in tertiary structure stabilize the quaternary structure.
• Quaternary structure leads to either globular or fibrous proteins.
• Globular proteins have hydrophilic amino acids on the outside and hydrophobic amino acids on the inside, making them soluble in water.
• Examples of globular proteins include hemoglobin and insulin.
• Fibrous proteins have unique structures that give them specific mechanical properties.

Protein Structure and Function

• A functional protein requires one or more polypeptides folded correctly.
• Incorrect folding can lead to diseases like Alzheimer's and Parkinson's.
• A protein's function is directly related to its three-dimensional shape (conformation).
• Proteins have four hierarchical levels of structural organization: primary, secondary, tertiary, and quaternary.

Primary Structure

• The primary structure is the linear sequence of amino acids in a protein.
• It is determined by genetic information encoded in DNA, which is transcribed into mRNA and then translated into protein.
• The primary structure is crucial for a protein's function, and even a single amino acid substitution can cause a protein to malfunction.
• Sickle cell disease is an example where a valine amino acid substitutes for glutamic acid at position 6, leading to a dysfunctional protein and the condition of sickle cell anemia.

Secondary Structure

• Secondary structure refers to the patterns of hydrogen bonds formed between amino acids within a polypeptide chain.
• These bonds stabilize the polypeptide and contribute to the overall shape.
• Two main types of secondary structures are alpha helices and beta pleated sheets.

Alpha Helix Formation

• In an alpha helix, the CO group of one amino acid forms a hydrogen bond with the NH group of an amino acid four residues away (n+4).
• This pattern creates a helical structure that is stabilized by the hydrogen bonds.

Beta Pleated Sheets

• Beta pleated sheets are formed by hydrogen bonds between adjacent polypeptide chains or segments of the same chain that are folded back on themselves.
• The polypeptide chains or segments align side-by-side, forming a pleated sheet-like structure.
• The strong structural properties of silk are a result of its beta pleated sheet structure, which contributes to its strength.

Protein Types and Functions

• Enzymes: Catalyze biochemical reactions.
• Hormonal Proteins: Regulate physiological processes.
• Receptor Proteins: Bind to signal molecules and trigger cellular responses.
• Sensory Proteins: Detect environmental stimuli like light, taste, or smell.
• Storage Proteins: Store nutrients or other molecules.
• Structural Proteins: Provide support and shape to cells and tissues.
• Transport Proteins: Move molecules across cell membranes or throughout the body.

Amino Acids and Protein Synthesis

• Proteins are polymers made up of monomers called amino acids.
• Amino acids have a central carbon atom bonded to an amino group (NH2), a carboxyl group (COOH), and a variable R group.
• There are 20 different amino acids that can be found in proteins.
• The R groups of amino acids determine their unique characteristics.
• R groups can be nonpolar, polar, or charged.
• Amino acids are linked together by peptide bonds formed through condensation reactions, releasing a water molecule.
• The order of amino acids in a polypeptide chain determines its primary structure.
• The N-terminus of a polypeptide chain has a free amino group (NH2), while the C-terminus has a free carboxyl group (COOH).