🔬Biological Chemistry I Unit 3 – Amino Acids and Protein Structure

What Are Amino Acids?

• Building blocks of proteins consist of an amino group, carboxyl group, and a unique side chain (R group) attached to a central alpha carbon
• Amino acids are zwitterions at physiological pH due to the presence of both positively charged amino groups and negatively charged carboxyl groups
• Can exist in either L or D stereoisomeric forms, but only L-amino acids are found in proteins
• Play crucial roles in various biological processes, including neurotransmission (glycine, glutamate), hormone synthesis (tyrosine), and energy metabolism (leucine, isoleucine, valine)
• Serve as precursors for the synthesis of other important biomolecules, such as nucleotides, hormones, and neurotransmitters
• Classified as essential (must be obtained through diet) or non-essential (can be synthesized by the body) based on the body's ability to synthesize them
• Participate in various metabolic pathways, including the urea cycle, which helps remove excess nitrogen from the body

The 20 Standard Amino Acids

• There are 20 standard amino acids commonly found in proteins, each with a unique side chain (R group) that determines its specific properties
• Glycine is the simplest amino acid, with a hydrogen atom as its side chain, while proline has a unique cyclic structure that introduces kinks in protein structures
• Amino acids can be classified based on the properties of their side chains, such as polarity, charge, and hydrophobicity
  • Polar amino acids include serine, threonine, cysteine, asparagine, and glutamine
  • Charged amino acids include lysine (positive), arginine (positive), histidine (positive at acidic pH), aspartate (negative), and glutamate (negative)
  • Hydrophobic amino acids include alanine, valine, leucine, isoleucine, methionine, phenylalanine, tyrosine, and tryptophan
• Some amino acids have special roles in protein structure and function, such as cysteine, which can form disulfide bonds, and glycine, which provides flexibility in protein structures
• The genetic code determines the specific sequence of amino acids in a protein, with each amino acid encoded by one or more codons (triplets of nucleotides)
• Rare amino acids, such as selenocysteine and pyrrolysine, can be incorporated into proteins through unique translational mechanisms

Amino Acid Properties and Classification

• Amino acids are classified based on the properties of their side chains, which influence protein structure, function, and interactions
• Polarity: Amino acids can be polar (hydrophilic) or non-polar (hydrophobic) depending on the presence of functional groups that can form hydrogen bonds with water
  • Polar amino acids (serine, threonine, cysteine, asparagine, glutamine) are often found on the surface of proteins, interacting with the aqueous environment
  • Non-polar amino acids (alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan) are usually buried within the protein core, contributing to protein stability through hydrophobic interactions
• Charge: Amino acids can be positively charged (lysine, arginine, histidine), negatively charged (aspartate, glutamate), or neutral (all others) at physiological pH
  • Charged amino acids participate in ionic interactions and salt bridges, which contribute to protein stability and specific interactions with other molecules
• Aromaticity: Amino acids with aromatic side chains (phenylalanine, tyrosine, tryptophan) can participate in pi-stacking interactions and contribute to protein stability and ligand binding
• Size: Amino acids vary in size, from the smallest (glycine) to the largest (tryptophan), which influences protein packing and folding
• Special properties: Some amino acids have unique properties, such as cysteine's ability to form disulfide bonds, proline's role in introducing kinks in protein structures, and glycine's conformational flexibility

Peptide Bonds and Primary Structure

• Amino acids are joined together by peptide bonds, which are formed through a condensation reaction between the carboxyl group of one amino acid and the amino group of another
• The resulting linear sequence of amino acids linked by peptide bonds is called the primary structure of a protein
• Peptide bonds have a partial double bond character due to resonance, which restricts rotation around the bond and introduces planarity to the peptide backbone
• The primary structure of a protein is determined by the genetic code, which specifies the sequence of amino acids in the polypeptide chain
• The N-terminus of a polypeptide chain refers to the end with a free amino group, while the C-terminus refers to the end with a free carboxyl group
• The primary structure of a protein is crucial for determining its higher-order structures (secondary, tertiary, and quaternary) and ultimately its function
• Mutations in the primary structure can lead to changes in protein folding, stability, and function, potentially resulting in disease states (sickle cell anemia, cystic fibrosis)

Protein Secondary Structures

• Secondary structures are local, regular arrangements of amino acids in a polypeptide chain, stabilized by hydrogen bonds between the peptide backbone atoms
• The two main types of secondary structures are alpha helices and beta sheets
  • Alpha helices are right-handed spiral conformations, with hydrogen bonds formed between the carbonyl oxygen of one amino acid and the amino hydrogen of the amino acid four residues ahead in the sequence
  • Beta sheets are extended conformations with hydrogen bonds formed between peptide backbones of adjacent strands, which can be parallel (N-termini aligned) or antiparallel (N-terminus of one strand aligned with C-terminus of the other)
• Other secondary structure elements include turns and loops, which connect alpha helices and beta sheets and contribute to protein flexibility and function
• The formation of secondary structures is influenced by the amino acid sequence, with certain amino acids having a higher propensity for forming specific structures (alanine, leucine for alpha helices; valine, isoleucine, threonine for beta sheets)
• Secondary structures are important for protein folding, stability, and function, as they help to minimize the exposure of hydrophobic amino acids to the aqueous environment and provide a framework for higher-order structures
• Disruption of secondary structures by mutations or environmental factors (pH, temperature) can lead to protein misfolding and aggregation, which are associated with various diseases (Alzheimer's, Parkinson's)

Tertiary and Quaternary Structures

• Tertiary structure refers to the three-dimensional arrangement of a single polypeptide chain, resulting from interactions between amino acid side chains
  • Stabilizing forces include hydrophobic interactions, hydrogen bonds, ionic interactions, and disulfide bonds
  • The tertiary structure is crucial for protein function, as it determines the spatial arrangement of functional groups and binding sites
• Quaternary structure refers to the arrangement of multiple polypeptide chains (subunits) in a multi-subunit protein complex
  • Subunits can be identical (homooligomers) or different (heterooligomers), and their association is stabilized by the same types of interactions as in tertiary structure
  • Quaternary structure allows for the formation of large, complex proteins with multiple functions, such as hemoglobin (oxygen transport) and DNA polymerase (DNA replication)
• Protein domains are independently folding units within a polypeptide chain that often have specific functions, such as catalysis or ligand binding
  • Domains can be combined in different ways to create proteins with diverse functions
• Conformational changes in tertiary and quaternary structures can occur in response to ligand binding, post-translational modifications, or environmental factors, regulating protein activity and function (allosteric regulation)
• Disruption of tertiary or quaternary structures by mutations or environmental factors can lead to protein dysfunction and disease (sickle cell anemia, prion diseases)

Protein Folding and Stability

• Protein folding is the process by which a polypeptide chain acquires its native three-dimensional structure, which is essential for its function
• The folding process is driven by the minimization of free energy, with the native state representing the most thermodynamically stable conformation
• Protein folding is guided by the amino acid sequence (primary structure), which determines the local interactions and the overall energy landscape of the folding process
• The folding process can be described by various models, such as the hydrophobic collapse model (initial rapid collapse driven by hydrophobic interactions) and the nucleation-condensation model (formation of local secondary structures followed by global folding)
• Chaperones are proteins that assist in the folding process by preventing aggregation, facilitating proper folding, and rescuing misfolded proteins
  • Examples include Hsp70 (heat shock protein 70) and GroEL/GroES (chaperonin system in bacteria)
• Protein stability is influenced by various factors, such as temperature, pH, ionic strength, and the presence of denaturants (urea, guanidinium chloride)
  • Proteins can be denatured by extreme conditions, leading to the loss of native structure and function
  • Denaturation is often reversible, with proteins able to refold into their native state upon removal of the denaturing agent
• Misfolding and aggregation of proteins are associated with various diseases, such as Alzheimer's (amyloid-beta), Parkinson's (alpha-synuclein), and Huntington's (huntingtin) diseases
  • Strategies to prevent or reverse protein misfolding and aggregation are potential therapeutic approaches for these diseases

Analyzing Protein Structure

• Various experimental techniques are used to determine protein structure, each with its own advantages and limitations
• X-ray crystallography is the most widely used method for determining high-resolution protein structures
  • Involves crystallizing the protein, exposing the crystal to X-rays, and analyzing the diffraction pattern to determine the electron density map and atomic coordinates
  • Provides detailed information about the positions of individual atoms, but requires the protein to form well-ordered crystals
• Nuclear magnetic resonance (NMR) spectroscopy is used to determine protein structures in solution
  • Relies on the magnetic properties of atomic nuclei (usually 1H, 13C, and 15N) and their interactions to generate distance and angle restraints used to calculate the protein structure
  • Provides information about protein dynamics and conformational flexibility, but is limited to relatively small proteins (<50 kDa)
• Cryo-electron microscopy (cryo-EM) is an emerging technique for determining the structures of large protein complexes and macromolecular assemblies
  • Involves rapidly freezing the sample in vitreous ice, imaging it using an electron microscope, and reconstructing the 3D structure from multiple 2D projections
  • Allows for the determination of structures in near-native states, but typically has lower resolution compared to X-ray crystallography
• Computational methods, such as homology modeling and ab initio prediction, can be used to generate protein structure models when experimental data is unavailable
  • Homology modeling relies on the evolutionary relationship between proteins, using the structure of a homologous protein as a template to model the target protein
  • Ab initio prediction attempts to predict protein structure based solely on the amino acid sequence, using physical and statistical principles to explore the conformational space
• Analyzing protein structure is crucial for understanding protein function, designing drugs (structure-based drug design), and engineering proteins with novel or enhanced properties (protein engineering)