🔬Biological Chemistry I Unit 4 – Protein Folding: Structure and Function

Protein Basics: Building Blocks and Primary Structure

• Proteins consist of amino acids linked together by peptide bonds forming polypeptide chains
• 20 different amino acids serve as the building blocks of proteins each with unique side chains (R groups) that determine their properties
• Amino acids contain an amino group ($NH_2$), a carboxyl group ($COOH$), and a specific R group attached to the central alpha carbon ($C_\alpha$)
• The sequence of amino acids in a protein's polypeptide chain is determined by the genetic code and is known as its primary structure
  • The genetic code is read in triplets of nucleotides called codons each corresponding to a specific amino acid
• Peptide bonds form through a condensation reaction between the carboxyl group of one amino acid and the amino group of another releasing a water molecule
• The N-terminus of a polypeptide chain has a free amino group while the C-terminus has a free carboxyl group
• Disulfide bonds can form between cysteine residues stabilizing the protein's structure

Secondary Structure: Alpha Helices and Beta Sheets

• Secondary structure refers to the local folding of a polypeptide chain into regular repeating patterns stabilized by hydrogen bonds
• The two main types of secondary structure are alpha helices and beta sheets
• Alpha helices are right-handed coiled structures stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amino hydrogen of another amino acid located four residues ahead in the sequence
  • Alpha helices have 3.6 amino acid residues per turn and a pitch of 5.4 Å
  • The R groups of amino acids in an alpha helix point outward from the helical axis
• Beta sheets are formed by multiple polypeptide chains or segments of a single chain lying side-by-side stabilized by hydrogen bonds between the carbonyl oxygen of one strand and the amino hydrogen of the adjacent strand
  • Beta sheets can be parallel (strands running in the same direction) or antiparallel (strands running in opposite directions)
  • The R groups of amino acids in a beta sheet alternate above and below the plane of the sheet
• Turns and loops are short irregular structures that connect alpha helices and beta sheets allowing the polypeptide chain to change direction

Tertiary Structure: 3D Folding and Interactions

• Tertiary structure refers to the three-dimensional arrangement of a polypeptide chain resulting from interactions between the side chains of amino acids
• Hydrophobic interactions play a crucial role in tertiary structure formation as nonpolar amino acids tend to cluster in the interior of the protein away from the aqueous environment
• Hydrogen bonds between side chains contribute to the stability of tertiary structure
• Ionic interactions (salt bridges) can form between positively and negatively charged side chains stabilizing the protein's folded state
• Disulfide bonds between cysteine residues can further stabilize the tertiary structure
• The tertiary structure of a protein determines its unique shape and function
  • For example, the tertiary structure of enzymes creates specific active sites that bind substrates and catalyze reactions
• Chaperone proteins assist in the proper folding of other proteins preventing aggregation and misfolding
• Protein domains are independently folded regions of a protein that can have distinct functions and evolutionary origins

Quaternary Structure: Multi-Subunit Proteins

• Quaternary structure refers to the arrangement of multiple polypeptide chains (subunits) in a multi-subunit protein
• Subunits can be identical (homooligomers) or different (heterooligomers) and are held together by noncovalent interactions such as hydrogen bonds, ionic interactions, and hydrophobic interactions
  • Hemoglobin is a heterotetramer consisting of two alpha and two beta subunits each with an oxygen-binding heme group
• Quaternary structure allows for cooperative effects and allosteric regulation where the binding of a ligand to one subunit affects the activity of other subunits
  • In hemoglobin, oxygen binding to one subunit increases the affinity of the other subunits for oxygen facilitating efficient oxygen transport
• Subunit arrangement can create channels, pores, or active sites at the interfaces between subunits
• Changes in pH, temperature, or ligand binding can induce conformational changes in the quaternary structure modulating protein function
• Studying the quaternary structure of proteins provides insights into their biological roles and regulation

Factors Influencing Protein Folding

• Protein folding is a complex process influenced by various factors that determine the final three-dimensional structure of a protein
• The amino acid sequence (primary structure) contains all the information necessary for a protein to fold into its native state
• Hydrophobic interactions are a major driving force in protein folding as nonpolar amino acids tend to cluster in the interior of the protein minimizing their contact with water
• Hydrogen bonding between backbone atoms and side chains stabilizes secondary structures (alpha helices and beta sheets) and contributes to the overall folding process
• Disulfide bonds formed between cysteine residues can stabilize the folded state of a protein
• Chaperone proteins assist in the folding process by preventing aggregation and promoting proper folding
  • Examples of chaperones include Hsp60 (GroEL in bacteria) and Hsp70
• Crowding in the cellular environment can influence protein folding by increasing the effective concentration of proteins and favoring compact folded states
• Post-translational modifications such as glycosylation and phosphorylation can affect protein folding and stability
• Environmental factors such as temperature, pH, and ionic strength can impact the folding process and the stability of the native state

Protein Misfolding and Diseases

• Protein misfolding occurs when a protein fails to achieve or maintain its native three-dimensional structure leading to loss of function and potentially harmful aggregation
• Misfolded proteins can expose hydrophobic regions that are normally buried in the interior leading to aggregation and the formation of insoluble deposits
• Protein misfolding is associated with various diseases known as proteinopathies or protein conformational disorders
  • Alzheimer's disease is characterized by the accumulation of misfolded amyloid-beta peptides forming plaques in the brain
  • Parkinson's disease involves the aggregation of misfolded alpha-synuclein proteins in neurons
  • Huntington's disease is caused by an expanded polyglutamine tract in the huntingtin protein leading to misfolding and aggregation
• Prion diseases such as Creutzfeldt-Jakob disease (CJD) and bovine spongiform encephalopathy (BSE) are caused by the misfolding and aggregation of prion proteins
• Chaperone proteins and protein quality control systems help prevent and mitigate the effects of protein misfolding
  • The ubiquitin-proteasome system targets misfolded proteins for degradation
  • Autophagy pathways can degrade larger protein aggregates and damaged organelles
• Studying the mechanisms of protein misfolding and its associated diseases can lead to the development of therapeutic strategies such as small molecules that stabilize native protein conformations or enhance protein clearance pathways

Techniques for Studying Protein Structure

• Various techniques are used to determine and analyze the structure of proteins at different levels of resolution
• X-ray crystallography involves crystallizing proteins and exposing them to X-rays to generate diffraction patterns that can be used to determine the three-dimensional structure at atomic resolution
  • Requires the production of high-quality protein crystals which can be challenging for some proteins
  • Provides detailed information about the positions of individual atoms in the protein
• Nuclear magnetic resonance (NMR) spectroscopy uses the magnetic properties of atomic nuclei to determine the structure of proteins in solution
  • Provides information about the dynamics and flexibility of proteins
  • Limited to relatively small proteins (< 50 kDa)
• Cryo-electron microscopy (cryo-EM) involves freezing protein samples in vitreous ice and imaging them using an electron microscope
  • Allows the determination of protein structures at near-atomic resolution without the need for crystallization
  • Particularly useful for large protein complexes and membrane proteins
• Circular dichroism (CD) spectroscopy measures the differential absorption of left and right circularly polarized light by proteins providing information about their secondary structure content
• Fluorescence spectroscopy can be used to study protein folding and dynamics by monitoring the intrinsic fluorescence of tryptophan residues or using extrinsic fluorescent probes
• Computational methods such as molecular dynamics simulations and protein structure prediction algorithms complement experimental techniques in studying protein structure and dynamics

Protein Function and Its Relationship to Structure

• The function of a protein is intimately linked to its three-dimensional structure which is determined by its amino acid sequence
• Enzymes are proteins that catalyze chemical reactions by lowering the activation energy
  • The active site of an enzyme is a specific region where the substrate binds and the reaction takes place
  • The structure of the active site is complementary to the transition state of the reaction allowing for specific substrate binding and catalysis
• Transport proteins such as hemoglobin and ion channels have structures that facilitate the binding and movement of specific molecules or ions across membranes
• Structural proteins like collagen and keratin have repetitive sequences and structures that provide mechanical support and stability to tissues
• Regulatory proteins such as transcription factors and protein kinases have structures that allow them to bind specific DNA sequences or phosphorylate target proteins modulating cellular processes
• Signaling proteins like G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) have structures that enable them to bind extracellular ligands and transmit signals across the cell membrane
• Motor proteins such as myosin and kinesin have structures that allow them to convert chemical energy (ATP hydrolysis) into mechanical motion
• Antibodies (immunoglobulins) have a characteristic Y-shaped structure with variable regions that bind specific antigens and constant regions that mediate immune responses
• Understanding the relationship between protein structure and function allows for the prediction of function based on sequence and structure, the design of proteins with novel functions, and the development of targeted therapies for diseases involving protein dysfunction