Protein Structure: Secondary, Tertiary, Quaternary Levels, and Folding Mechanisms
Secondary Structures of Proteins
The $\alpha$-Helix
The $\alpha$-helix is a common secondary structure.
It is stabilized by hydrogen bonds that form between carbonyl () groups and amide () groups located in the polypeptide backbone.
The $\beta$-Sheet
Structure: In a $\beta$-sheet, the polypeptide chain folds back and forth on itself, creating a pleated, sheet-like structure.
Stabilization: This structure is stabilized by hydrogen bonds formed between carbonyl groups in one polypeptide chain and amide groups in an adjacent polypeptide chain across the way.
R-Group Orientation: The R groups (side chains) of the amino acids alternately project above and below the plane of the $\beta$-sheet.
Composition: $\beta$-sheets typically consist of to polypeptide chains or segments aligned side by side.
Hydrogen Bonding Details: In each chain, amide groups form hydrogen bonds with carbonyl groups on either side, except for the groups located at the ends of each strand.
Strand Directionality:
Parallel: Adjacent strands run in the same direction (from amino end to carboxyl end).
Antiparallel: Adjacent strands run in opposite directions. This configuration is more stable because the carbonyl and amide groups are more optimally aligned for hydrogen bonding.
Notation: $\beta$-sheets are commonly represented by broad arrows, where the direction of the arrow indicates the polypeptide segment's orientation from the amino () end to the carboxyl () end. Figure depicts an antiparallel $\beta$-sheet with arrows running in opposite directions.
Tertiary Structure of Proteins
Definition and Formation
Definition: Tertiary structure refers to the unique three-dimensional conformation of a single polypeptide chain.
Composition: It is typically formed by the folding and arrangement of multiple secondary structure elements (e.g., $\alpha$-helices and $\beta$-sheets) within that single chain.
Driving Forces: The formation of tertiary structure is largely dictated by interactions between the amino acid R groups (side chains).
In contrast, secondary structures primarily depend on backbone interactions and are relatively independent of R groups.
Key Interactions: Tertiary structure is determined by:
The spatial distribution of hydrophilic (water-loving) and hydrophobic (water-fearing) R groups along the molecule.
Various types of chemical bonds and interactions that form between different R groups, including:
Ionic bonds
Hydrogen bonds
van der Waals interactions
Conformational Flexibility: Amino acids whose R groups interact to form bonds may be far apart in the linear primary sequence of the polypeptide chain but come into close proximity when the protein folds. Tertiary structures often include loops or turns in the backbone to allow these distant R groups to interact in space.
Models of Protein Structure
Ball-and-Stick Model (Fig. ): Highlights individual atoms within the amino acid chain.
Ribbon Model (Fig. ): Emphasizes secondary structures, depicting $\alpha$-helices as twisted ribbons and $\beta$-sheets as broad arrows.
Space-Filling Model (Fig. ): Shows the overall shape and contour of the folded protein.
Structure Determines Function
Hierarchy of Structure: The sequence of amino acids (primary structure) dictates the formation of secondary and tertiary structures.
Functional Basis: Tertiary structure is critical for protein function because the molecule's three-dimensional shape—including its surface contours, charge distribution, and internal binding pockets—enables it to perform specific roles (e.g., structural support, membrane channel, enzyme, signaling molecule).
Example: A bacterial protein (Fig. ) with a central cavity can bind a specific small molecule through hydrogen bonds formed by certain R groups, illustrating how shape facilitates binding and function.
Denaturation and Loss of Function
Denaturation: This is the process where a protein loses its native three-dimensional structure, leading to a loss of its functional activity.
Causes: Most proteins can be denatured by:
Chemical treatment: Disrupts hydrogen and ionic bonds.
High temperature: Also disrupts hydrogen and ionic bonds.
Consequence: When denatured, proteins lose their specific functional activity.
Impact of Mutations: Mutant proteins with an altered amino acid sequence that prevents proper folding are often inactive or function improperly, further demonstrating the link between structure and function.
The Primary Sequence Dictates Protein Folding: Anfinsen's Experiment (How Do We Know?)
Background
While natural selection acts on the function derived from a protein's folded secondary and tertiary structure, only the primary structure (amino acid sequence) is directly encoded by a gene.
This apparent paradox was resolved by experiments demonstrating that the complex folded structures are inherently determined by the primary amino acid sequence.
Experiment Design (Christian B. Anfinsen, )
Subject: Ribonuclease A, a relatively small enzyme composed of amino acids.
Key Chemicals:
Urea: Forms stronger hydrogen bonds than water, effectively disrupting the hydrogen bonds that stabilize polypeptide chains, as well as the protein's hydrophobic core.
2-Mercaptoethanol (ME): A mild reducing agent that breaks the relatively weak covalent disulfide bonds () which link cysteine side chains in ribonuclease A.
Goal: To observe the enzyme's activity after introducing and removing these denaturing agents.
Results and Interpretation (Fig. )
Treatment with Urea + ME (Fig. ):
Effect: Both disulfide bonds were broken, and hydrogen bonds/hydrophobic interactions were disrupted, leading to complete unfolding (denaturation) of ribonuclease A.
Activity: The enzyme completely lost its activity.
Removal of ME, Retention of Urea (Fig. ):
Effect: Disulfide bonds formed incorrectly (misfolding) because the urea preventedthe protein from adopting its proper overall three-dimensional shape.
Activity: The misfolded enzyme remained inactive.
**Removal of Urea, Retention of Trace ME (Fig. ):
Effect: With urea removed, hydrogen bonds and hydrophobic interactions could re-form correctly, guiding the protein towards its native fold. The trace amount of ME allowed incorrect disulfide bonds to break and re-form with the proper partners, facilitating correct folding.
Activity: The protein regained its full activity.
Conclusion and Legacy
Conclusion: These findings strongly supported the hypothesis that the primary amino acid sequence intrinsically dictates how a protein folds into its secondary and tertiary structures.
Spontaneous Folding: Under normal cellular conditions, protein folding often occurs spontaneously, driven towards a state of minimum free energy.
Nobel Prize: For these groundbreaking experiments, Anfinsen was awarded a Nobel Prize in .
Follow-up Work: The study of protein folding led to the discovery of chaperone proteins that assist the folding of some proteins and the understanding of diseases (e.g., Alzheimer's, Parkinson's, mad cow disease) caused by misfolded protein aggregates.
Quaternary Structure of Proteins
Definition
Higher-Order Structure: Quaternary structure exists in proteins composed of two or more separate polypeptide chains (subunits), each possessing its own tertiary structure, that come together to form a larger, functional complex.
Functional Dependence: The activity of a multi-subunit protein complex is dependent on the quaternary structure formed by the association of its various tertiary-structured subunits.
Subunit Composition
Identical Subunits (Fig. ): Some proteins, like an enzyme from HIV, consist of multiple identical polypeptide subunits.
Different Subunits (Fig. ): Many proteins, such as hemoglobin, are composed of different types of subunits (e.g., in hemoglobin, two $\alpha$ subunits and two $\beta$ subunits).
Functional Implications
Subunit Interaction: The polypeptide subunits within a quaternary structure can subtly influence each other, impacting their overall function.
Example (Hemoglobin): Hemoglobin, which transports oxygen in red blood cells, has subunits. When one subunit binds to oxygen, it induces a slight conformational change that is transmitted to the other subunits, making it easier for them to also bind oxygen. This cooperative binding significantly improves oxygen transport from the lungs to the tissues.
Chaperone Proteins
Role in Protein Folding
Primary Structure as Determinant: The amino acid sequence (primary structure) is the fundamental determinant of a protein's secondary, tertiary, and, if applicable, quaternary structures.
Folding Dynamics: Approximately of proteins fold very rapidly (within milliseconds) as they are being synthesized.
Challenge of Slow Folding: Some proteins fold more slowly. During the longer period these polypeptides remain in an unfolded (denatured) state, their hydrophobic groups are exposed to the crowded cellular cytoplasm.
Risk of Aggregation: The hydrophobic effect and van der Waals interactions tend to cause these exposed hydrophobic groups to improperly aggregate with other macromolecules, which can prevent proper folding.
Risk for Denatured Proteins: Even correctly folded proteins can unfold due to stress (e.g., elevated temperature) and, in their denatured state, become susceptible to inappropriate aggregation.
Mechanism of Action
Cellular Protection: Cells have evolved specialized proteins called chaperones to protect slow-folding or denatured proteins.
Binding and Shielding: Chaperones bind to the exposed hydrophobic and nonpolar R groups of unfolded polypeptides, shielding them from inappropriate aggregation with other molecules.
Facilitating Proper Folding: Through repeated cycles of binding and release, chaperones provide the polypeptide with sufficient time and an appropriate environment to find and attain its correct three-dimensional structure.
Clinical Significance
Disease Association: Many diseases are linked to improperly folded protein aggregates, highlighting the critical role of accurate protein folding and chaperone function.
Examples include Alzheimer's disease, Parkinson's disease, and mad cow disease (bovine spongiform encephalopathy).