Lecture 2
Introduction to Protein Organization
Proteins are modular and have distinct domains.
Major structural levels of proteins:
Primary structure (1o): Amino acid (AA) sequence.
Secondary structure (2o): Forms of alpha helices and beta sheets.
Tertiary structure (3o): Full 3-D conformation.
Quaternary structure (4o): Complex formed by more than one polypeptide chain.
Protein Folding
Protein folding is crucial for functionality.
Folded by chaperones, particularly heat shock proteins (hsp70, hsp60).
Folding pathways to know:
Hsp70: binds to nascent proteins; uses ATP hydrolysis to aid folding.
Hsp60: helps to refold misfolded proteins by encapsulating them.
Importance of Protein Shape
Proteins function optimally when they maintain the correct shape.
Environmental conditions are vital to maintain this conformation.
Disruption in protein shape can lead to diseases, often associated with amyloids (e.g., Alzheimer's, Parkinson's).
Noncovalent Interactions in Folding
Protein folding is largely dependent on weak noncovalent bonds that involve:
Hydrogen bonds.
Electrostatic attractions.
Van der Waals attractions.
Definitions:
Ionic bond: Electrostatic interaction between two fully charged atoms.
Hydrogen bond: Interaction between a proton and an electronegative atom within molecules.
Van der Waals: Weak forces due to fluctuations in electron distribution in atoms.
Levels of Organization in Proteins
Proteins exhibit multiple levels of organization:
1o: Amino acid sequence.
2o: Stretches forming alpha-helices or beta-sheets.
3o: Full 3-D structure.
4o: Complex of more than one polypeptide.
Alpha Helices
The alpha helix is a predominant secondary structure critical for membrane transport.
H-bonding occurs between every fourth residue (n=4):
C=O of one peptide bond links to N-H of another.
The structure is polar, with a positive N-terminus and a negative C-terminus.
Beta Sheets
Formed by hydrogen bonding between strands, producing two types:
Anti-parallel beta sheets: Strands run in alternating directions; stronger due to well-aligned H-bonds.
Parallel beta sheets: Strands run in the same N-C direction; weaker due to longer stretches separating them.
Disease-Related Proteins: Amyloids
Diseases linked to amyloids include:
Alzheimer’s disease (amyloid plaques).
Parkinson’s disease (Lewy bodies).
Prion diseases (CJD).
Amyloid fibrils are stable beta-sheet aggregates that can propagate and lead to cellular dysfunction.
Folding Mechanisms Inside the Cell
The cytosol is a reducing environment with no disulfide bridges forming.
The folding pathway includes molten globule intermediates, which are less ordered but have some secondary structure.
Chaperones in Protein Folding
Chaperones, such as hsp70 and hsp60, facilitate correct protein folding and prevent misfolding.
hsp70: Binds nascent polypeptides; ATP-dependent cycle for stabilization.
hsp60: Encapsulates misfolded proteins for correct refolding, typically after a protein has exited the ribosome.
Clinical Relevance of Chaperones and Misfolding
Abnormalities in chaperone function are implicated in cancer biology.
Drugs targeting heat shock proteins are under investigation for therapeutic use.
Examples include geldanamycin derivatives.
Conclusion: Folding and Disease
Proper protein folding is essential; misfolding can cause severe diseases.
Understanding protein organization and folding pathways is critical in biochemistry and therapeutic development.
Additional Notes
For more recent therapeutic developments, see additional sections on specific drug trials related to Heat Shock Proteins (HSPs).
References
Molecular Biology of the Cell (MBC), 7th Edition for figures and detailed descriptions of protein structures.
Introduction to Protein Organization
Proteins are modular and often contain distinct functional domains.
Major structural levels of proteins describe their complex three-dimensional arrangements:
Primary structure (1o): Refers to the linear sequence of amino acids (AAs) linked by covalent peptide bonds. This sequence dictates all higher-order structures.
Secondary structure (2o): Involves localized, regular folding patterns of the polypeptide backbone, primarily stabilized by hydrogen bonds. The two most common forms are alpha helices and beta sheets.
Tertiary structure (3o): Represents the full, intricate 3-D conformation of a single polypeptide chain, including the arrangement of all its secondary structures and the interactions between amino acid side chains. It is stabilized by various noncovalent interactions and sometimes disulfide bonds.
Quaternary structure (4o): Describes the complex formed by the association of more than one polypeptide chain (subunits) to form a functional multimeric protein. These subunits are held together by noncovalent interactions.
Protein Folding
Protein folding is a crucial and highly regulated process by which a newly synthesized polypeptide chain acquires its specific three-dimensional functional structure. This process is essential for its biological activity.
Proteins are correctly folded by specialized protein machinery called chaperones, particularly heat shock proteins (e.g., hsp70, hsp60).
Key folding pathways involving chaperones:
Hsp70 family: Binds to nascent polypeptides as they emerge from the ribosome, recognizing hydrophobic patches on the unfolded protein. It uses ATP hydrolysis to drive conformational changes that stabilize the folding pathway and prevent aggregation, facilitating correct folding before the protein is fully synthesized.
Hsp60 (chaperonin): Acts later in the folding pathway or on misfolded proteins. It provides a protective, enclosed environment (often called an 'Anfinsen cage') for misfolded proteins to refold correctly, also using ATP hydrolysis to induce conformational changes that promote productive folding.
Importance of Protein Shape
Proteins function optimally when they maintain their precise, native three-dimensional shape. This conformation is critical for their specific roles, such as enzyme catalysis (active site formation), ligand binding, and structural integrity.
Critical environmental conditions such as pH, temperature, and ionic strength are vital to maintain this specific conformation. Deviations can lead to denaturation.
Disruption in protein shape or misfolding can lead to a loss of function, gain of toxic function, and severe diseases, often associated with the formation of insoluble protein aggregates called amyloids (e.g., Alzheimer's disease, Parkinson's disease, cystic fibrosis, prion diseases).
Noncovalent Interactions in Folding
Protein folding is largely driven and stabilized by a combination of weak noncovalent bonds and the hydrophobic effect. These interactions include:
Hydrogen bonds: These form between a partially positively charged hydrogen atom and a partially negatively charged electronegative atom (like oxygen or nitrogen). They are crucial in stabilizing secondary structures (backbone H-bonds) and contributing to tertiary and quaternary structures (side-chain H-bonds).
Electrostatic attractions (Ionic bonds): Occur between fully charged amino acid side chains (e.g., between a positively charged lysine and a negatively charged aspartate). These are strong interactions but can be sensitive to pH and ionic strength.
Van der Waals attractions: Weak, short-range attractive forces that arise from transient fluctuations in electron distribution around nonpolar atoms. These interactions are individually weak but collectively significant when many atoms are in close proximity within the tightly packed interior of a protein.
Hydrophobic effect: The primary driving force for protein folding. Nonpolar amino acid side chains tend to cluster together in the interior of the protein, away from the aqueous cellular environment, minimizing their contact with water and maximizing entropy of water molecules.
Levels of Organization in Proteins
Proteins exhibit a hierarchical multi-level organization:
1o: Defined solely by the specific linear sequence of amino acids.
2o: Local folding patterns, such as the alpha-helical and beta-sheet structures formed by hydrogen bonds within the polypeptide backbone.
3o: The overall three-dimensional arrangement of all atoms in a single polypeptide chain, including the spatial relationship of its secondary structures and side chains.
4o: Refers to the spatial arrangement of multiple polypeptide chains (subunits) in a complex protein, and the nature of their interactions.
Alpha Helices
The alpha helix is a common, rigid, and predominant secondary structure, often depicted as a rod-like coiled structure. It is particularly critical for membrane-spanning regions of proteins.
Key features:
It is a right-handed helix.
Stabilized by hydrogen bonding between the carbonyl oxygen (C=O) of one peptide bond and the amide hydrogen (N-H) of a peptide bond four residues away ( rule for H-bonding).
There are approximately amino acid residues per turn of the helix.
The pitch of the helix (the distance covered by one turn) is about nm ( Å).
Amino acid side chains project outwards from the helical backbone, minimizing steric hindrance.
The overall structure has a dipole moment, with a partial positive charge at the N-terminus and a partial negative charge at the C-terminus due to the alignment of peptide bond dipoles.
Beta Sheets
Beta sheets are formed by hydrogen bonding between adjacent polypeptide strands, creating a pleated, planar structure. They exhibit two main types:
Anti-parallel beta sheets: Strands run in alternating polypeptide directions (N-C then C-N). The hydrogen bonds between the strands are optimally aligned and nearly perpendicular to the strands, making this a very strong and stable configuration.
Parallel beta sheets: Strands run in the same N-C direction. The hydrogen bonds are oriented at an angle, making them slightly weaker and longer compared to anti-parallel sheets.
In both types, the amino acid side chains protrude alternately above and below the plane of the sheet.
Multiple strands can assemble to form large beta sheets, which are highly stable and can form the core structure of many proteins.
Disease-Related Proteins: Amyloids
Diseases linked to amyloid formation result from the misfolding and aggregation of specific proteins into insoluble fibrous deposits. These include:
Alzheimer’s disease: Characterized by the extracellular accumulation of amyloid-beta () peptides, forming amyloid plaques in the brain.
Parkinson’s disease: Associated with the intracellular aggregation of alpha-synuclein protein into Lewy bodies, primarily affecting neuronal cells.
Prion diseases (e.g., Creutzfeldt-Jakob Disease - CJD): Caused by the misfolding of normal cellular prion protein () into an infectious, aggregation-prone form () that can template further misfolding.
Amyloid fibrils are highly stable, elongated protein aggregates characterized by a cross- sheet structure. They are resistant to degradation and their accumulation can lead to cellular dysfunction, toxicity, and tissue damage, and are capable of self-propagation.
Folding Mechanisms Inside the Cell
The cytosol, the main intracellular fluid, maintains a reducing environment due to a high concentration of reducing agents like glutathione. This environment typically prevents the formation of stabilizing disulfide bridges between cysteine residues.
In contrast, disulfide bonds preferentially form in the oxidizing environment of the endoplasmic reticulum (ER lumen) in eukaryotes and the periplasmic space in prokaryotes, often catalyzed by enzymes like protein disulfide isomerase (PDI).
The intricate folding pathway often involves intermediate states, such as the 'molten globule,' which is characterized by a compact hydrophobic core and a significant amount of secondary structure, but with a less ordered tertiary structure that remains dynamic and flexible.
An initial step in folding is often rapid hydrophobic collapse, where nonpolar regions quickly coalesce to avoid water.
Chaperones in Protein Folding
Chaperones are a diverse family of proteins that assist in the correct folding of newly synthesized polypeptides, refold misfolded proteins, and prevent inappropriate protein aggregation, especially under stress conditions.
hsp70 (Heat Shock Protein 70):
Acts on nascent polypeptides as they are being synthesized on ribosomes or on partially unfolded proteins under stress.
It uses an ATP-dependent cycle: ATP binding leads to an open conformation with low substrate affinity, allowing binding to exposed hydrophobic patches; ATP hydrolysis (to ADP + Pi) causes a conformational change that tightens substrate binding; ADP/Pi release, often facilitated by Nucleotide Exchange Factors (NEFs), then allows for substrate release upon ATP binding.
hsp60 (Heat Shock Protein 60 - Chaperonin):
Forms a large, barrel-shaped oligomeric complex (e.g., GroEL/GroES in bacteria, mitochondrial hsp60).
Misfolded proteins are encapsulated within its central cavity. An ATP-dependent conformational change, often coupled with a 'lid' structure (e.g., GroES), provides a hydrophilic environment that promotes productive folding by transiently shielding the protein from aggregation.
Clinical Relevance of Chaperones and Misfolding
Abnormalities in chaperone function and protein misfolding are implicated in a wide range of human diseases:
Cancer biology: Many cancer cells overexpress certain heat shock proteins (e.g., hsp90, hsp70) which protect oncogenic proteins from degradation, thereby promoting cell survival and proliferation. Chaperone inhibition can destabilize these key cancer-promoting proteins.
Neurodegenerative diseases: In diseases like Alzheimer's and Parkinson's, chaperones attempt to clear toxic misfolded proteins, but their capacity can be overwhelmed, leading to aggregate accumulation and neurotoxicity.
Drugs targeting heat shock proteins are actively under investigation for therapeutic use:
Geldanamycin derivatives: These are inhibitors of hsp90 and have shown promise as anti-cancer agents by destabilizing client proteins essential for tumor growth.
Research is also exploring activators of chaperone function to enhance the clearance of misfolded proteins in neurodegenerative conditions.
Conclusion: Folding and Disease
Proper protein folding is exquisitely essential for all cellular processes, and misfolding can lead to severe pathologies.
A comprehensive understanding of protein organization, the complex folding pathways, and the crucial role of chaperones is critical for advancements in basic biochemistry, disease mechanisms, and the development of novel therapeutic strategies.
Additional Notes
For more recent therapeutic developments and clinical trial data on specific drug candidates targeting Heat Shock Proteins (HSPs) or related protein folding pathways, consult specialized journals and databases.
References
Molecular Biology of the Cell (MBC), 7th Edition, offers extensive figures and detailed descriptions of protein structures and folding mechanisms.