Foundations of Biochemistry: Proteins and Their Function

PointSolutions Session: BIOC 460/660 Foundations of Biochemistry and Molecular Biology I Lecture Notes

Chapter 4: The Three-Dimensional Structure of Proteins

Learning Goals

Understand the structure and properties of the peptide bond.
Explore the structural hierarchy in proteins.
Relate structure to function in fibrous proteins.
Analyze the structure of globular proteins.
Discuss protein folding and denaturation.

Importance of Amino Acid Sequences

Amino acid sequences provide critical insights into:
- 3D structure
- Function
- Cellular location
- Evolution
Consensus sequence: Reflects the most commonly occurring amino acid at each position in a given protein family.

Protein Families and Homologs

Homologs: Homologous proteins grouped within protein families.
- Paralogs: Homologous proteins within the same species.
- Orthologs: Homologous proteins found in different species.
Identification of homologs involves comparing sequences against a protein sequence database.

Evolutionary Insights from Protein Sequences

Organisms can be classified based on the sequence divergence of their protein families.
Signature sequences: Specific segments unique to certain taxonomic groups used to construct evolutionary trees.

Structure of Proteins

Unlike many organic polymers, proteins adopt distinct three-dimensional conformations, fulfilling biological functions; this is termed the native fold.
Key aspects of the native fold:
- Stability arises from a vast number of favorable interactions within the protein.
- There's an entropy cost associated with the folding process.

Favorable Interactions in Proteins

Hydrophobic effect: Water molecules are released from the solvation layer during folding, increasing net entropy.
Hydrogen bonds: Between N-H and C=O of peptide bonds, leading to regular secondary structures like α-helices and β-sheets.
Van der Waals/London dispersion forces: Provide medium-range attractions that contribute to interior protein stability.
Electrostatic interactions: Long-range attractions between charged groups, particularly stabilizing salt bridges in hydrophobic environments.

Hierarchical Structure Levels of Proteins

Primary structure: Sequence of amino acids.
Secondary structure: Localized arrangements such as α-helices and β-sheets.
Tertiary structure: Overall 3D arrangement of all atoms in a protein.
Quaternary structure: Assembly of multiple polypeptide subunits into a functional complex.

Peptide Bond Properties

The peptide bond is a resonance hybrid of two structures:
- Results in rigidity and nearly planar configuration.
- Possesses a strong dipole moment, favoring a trans configuration.
Peptide chains consist of planes around α-carbons, influencing secondary structure.

Ramachandran Plot

Displays favorable and unfavorable combinations of phi (Φ) and psi (Ψ) angles, reflecting probable dihedral angles found in proteins.
Provides insight into possible secondary structure arrangements.

Common Secondary Structures

α-Helix: Stabilized by hydrogen bonds between n and n+4 residues, forming a right-handed twist with approximately 3.6 residues per turn.
β-Sheet:
- Comprised of strands stabilized by hydrogen bonds. Strands may be parallel or antiparallel.

β-Turns

Occur when β-sheets change direction, forming a 180° turn over four amino acids stabilized by hydrogen bonds.
Proline and glycine are often found in β-turns due to their unique conformational properties.

Protein Stability and Folding

Proteins fold to their lowest energy conformations in a microsecond to second timeframe, addressing Levinthal's paradox through thermodynamic favorability of folding pathways.
Chaperones facilitate correct folding, preventing aggregation of misfolded proteins.

Protein Denaturation

Denaturation refers to the loss of structural integrity and function, influenced by:
- Heat
- Extreme pH
- Organic solvents
- Chaotropic agents like urea and guanidinium chloride.

Experimental Observations in Protein Folding

Ribonuclease Experiment: Demonstrated that amino acid sequence alone dictates refolding and the reformation of disulfide bonds when denatured and then restored.

Protein Structure Determination Techniques

X-ray crystallography: Allows visualization of three-dimensional structures, extensively developed since the 1950s with myoglobin.
NMR spectroscopy: Provides structural insights for solution-state proteins without crystallization.

Chapter 5: Protein Function

Learning Goals

Understand binding interactions of ligands and proteins.
Analyze quantitative models of protein-ligand interactions.
Explore interactions of globins with oxygen and other ligands.
Examine the mechanisms regulating muscle contraction.

Functionality of Globular Proteins

Significant functions include:
- Storage (e.g., myoglobin for oxygen).
- Transport (e.g., hemoglobin for gases).
- Immune defense (e.g., antibodies).
- Muscle movement (e.g., actin and myosin enzymes).
- Catalysis (e.g., lysozyme).

Ligand Binding Dynamics

Binding occurs reversibly as a transient process characterized by:
- Ligands: Small molecules that bind proteins, associated with specific binding sites.
- Rate constants governing interactions: Ka (association) and Kd (dissociation).
At equilibrium, Ka and Kd equalize, characterized by equilibrium constants (Ka or Kd).

Hill Equation and Cooperativity

The Hill equation facilitates understanding the cooperative binding of ligands:
- Provides insight into how proteins can adapt to physiological needs, increasing or decreasing affinity based on environmental conditions.
Cooperativity: Refers to the interaction between binding sites that can either facilitate or hinder subsequent binding of ligands.

Case Studies: Function of Globins

Myoglobin: Functions primarily as an oxygen storage protein.
Hemoglobin: Transport protein that binds oxygen cooperatively; dependent on pO2 levels in different tissues through the mechanisms of the Bohr effect.

Conclusion of Chapters 4 & 5

Summarizing key learnings:
- Structural determinants of protein functionality.
- Essential roles the protein folding pathway plays in maintaining biological activity.
Highlighting practical applications in biochemistry from understanding protein structure to function in living organisms and disease states.

To help you prepare for your exam, let's review the key concepts and terminology from Chapters 4 and 5 on protein structure, function, and ligand binding. This overview will address the definitions, usage in lectures, and applicability to problems for the topics you've outlined. The goal is to provide a comprehensive guide for your study.

Amino Acid Sequence and Importance

Amino acid sequence comparisons: The primary amino acid sequence is fundamental. Comparing sequences across different proteins or species allows for the identification of structural, functional, cellular location, and evolutionary relationships.
Consensus sequence: This is the most commonly occurring amino acid at each position in a given protein family, revealing crucial residues for structure and function.
Signature sequence: Specific segments unique to certain taxonomic groups, which are instrumental for constructing evolutionary trees and classifying organisms.
Homologs: Proteins that share a common evolutionary origin.
- Paralogs: Homologous proteins found within the same species, often arising from gene duplication, which may have diverged to perform different but related functions.
- Orthologs: Homologous proteins found in different species, which typically retain the same function across species.
Conserved substitutions: These are amino acid changes that maintain the physiochemical properties (e.g., polar to polar, nonpolar to nonpolar), thereby preserving protein structure and function.
Evolutionary relationships: The degree of sequence conservation directly correlates with evolutionary relatedness; highly conserved sequences suggest a more recent common ancestor or critical functional importance.

Protein Primary Sequence and Structure

Four levels of protein structure: This describes the hierarchical organization of proteins:
1. Primary structure: The linear sequence of amino acids linked by peptide bonds.
2. Secondary structure: Localized, regular folding patterns, such as $\alpha$ -helices and $\beta$ -sheets, stabilized by hydrogen bonds.
3. Tertiary structure: The overall three-dimensional arrangement of all atoms in a single polypeptide chain, including side chain interactions.
4. Quaternary structure: The assembly and arrangement of multiple polypeptide subunits (monomers) into a functional protein complex.
Bioenergetics and protein structure stability: Proteins fold into a specific, stable native fold, which is their lowest energy conformation. Stability is driven by a vast number of favorable non-covalent interactions, despite an entropy cost associated with ordering the polypeptide chain.
Forces stabilizing protein structure: These forces underpin the native fold:
- Hydrophobic effect: The primary driving force, where water molecules are released from the solvation layer around nonpolar groups, increasing the net entropy of the system. Predominates in the protein interior.
- Hydrogen bonds: Between backbone atoms (N-H and C=O of peptide bonds) forming secondary structures, and between side chains or side chain-backbone. Occur throughout the protein.
- Van der Waals/London dispersion forces: Weak, transient dipoles provide medium-range attractions, contributing to interior protein stability, especially in closely packed hydrophobic cores.
- Electrostatic interactions: Long-range attractions/repulsions between charged groups (ionic bonds or salt bridges), particularly stabilizing in low dielectric environments like the protein interior.
Resonance within a peptide bond: The peptide bond (C-N) exhibits partial double-bond character due to resonance stabilization between the carbonyl oxygen and the amide nitrogen. This results in the bond being rigid and planar, preventing rotation.
Rotational bonds and dihedral angles: The peptide bond itself does not rotate. Rotation is possible around the $\alpha$ -carbon, specifically at two bonds:
- Phi ( $\Phi$ ) angle: Rotation around the N– $\alpha$ C bond.
- Psi ( $\Psi$ ) angle: Rotation around the $\alpha$ C–C bond.
  These two dihedral angles dictate the conformation of the polypeptide backbone and are crucial for secondary structure formation.

Protein Secondary Structure

Ramachandran plots: These plots display the energetically favorable and unfavorable combinations of the $\Phi$ and $\Psi$ dihedral angles. They allow prediction of possible secondary structure motifs (e.g., $\alpha$ -helices, $\beta$ -sheets, left-handed helices) and identify sterically disallowed conformations.
Common secondary structures: The predominant forms are the $\alpha$ -helix and $\beta$ -sheet.
- $\alpha$ -Helix: A spiral conformation, typically right-handed, stabilized by hydrogen bonds between the carbonyl oxygen of residue $n$ and the amide hydrogen of residue $n+4$ . It contains approximately 3.6 residues per turn, with a pitch of 5.4 \unicode{x212B}. The diameter is about 1.2 \unicode{x212B} without side chains, or 2.3 \unicode{x212B} with side chains. Amino acid sequence influences stability (e.g., Proline disrupts, Glycine too flexible). Helices have a dipole moment (N-terminus positive, C-terminus negative), which can be stabilized by charged amino acids at the ends.
- $\beta$ -Sheet: Composed of multiple polypeptide strands ( $\beta$ -strands) typically in an extended conformation. These strands are stabilized by inter-strand hydrogen bonds between backbone atoms. Sheets can be parallel (N- to C-termini aligned) or antiparallel (alternating N-C and C-N).
$\beta$ -Turns: Occur when the polypeptide chain reverses direction, forming a 180 \unicode{x00B0} turn over four amino acid residues. Stabilized by a hydrogen bond between the carbonyl oxygen of residue $n$ and the amide hydrogen of residue $n+3$ . Proline (due to its cyclic structure restricting $\Phi$ angle) and Glycine (due to its conformational flexibility) are frequently found in $\beta$ -turns, especially at positions 2 and 3 respectively.
Circular dichroism (CD): This spectroscopic technique measures the differential absorption of left- and right-circularly polarized light by chiral molecules. It generates a characteristic spectrum for different secondary structures (e.g., distinct minima for $\alpha$ -helices and $\beta$ -sheets), allowing estimation of their fractional content within a protein or protein complex.

Protein Tertiary/Quaternary Structure

Tertiary structure: The full 3D arrangement of all atoms in a single polypeptide chain, stabilized by all previously mentioned non-covalent interactions (hydrophobic effect, H-bonds, van der Waals, electrostatic) and sometimes by covalent disulfide bonds. It defines the

Amino Acid Sequence and Importance

Amino acid sequence comparisons: The primary amino acid sequence is fundamental. Comparing sequences across different proteins or species allows for the identification of structural, functional, cellular location, and evolutionary relationships.
Consensus sequence: This is the most commonly occurring amino acid at each position in a given protein family, revealing crucial residues for structure and function.
Signature sequence: Specific segments unique to certain taxonomic groups, which are instrumental for constructing evolutionary trees and classifying organisms.
Homologs: Proteins that share a common evolutionary origin.
- Paralogs: Homologous proteins found within the same species, often arising from gene duplication, which may have diverged to perform different but related functions.
- Orthologs: Homologous proteins found in different species, which typically retain the same function across species.
Conserved substitutions: These are amino acid changes that maintain the physiochemical properties (e.g., polar to polar, nonpolar to nonpolar), thereby preserving protein structure and function.
Evolutionary relationships: The degree of sequence conservation directly correlates with evolutionary relatedness; highly conserved sequences suggest a more recent common ancestor or critical functional importance.

Protein Primary Sequence and Structure

Four levels of protein structure: This describes the hierarchical organization of proteins:
1. Primary structure: The linear sequence of amino acids linked by peptide bonds.
2. Secondary structure: Localized, regular folding patterns, such as $\alpha$ -helices and $\beta$ -sheets, stabilized by hydrogen bonds.
3. Tertiary structure: The overall three-dimensional arrangement of all atoms in a single polypeptide chain, including side chain interactions.
4. Quaternary structure: The assembly and arrangement of multiple polypeptide subunits (monomers) into a functional protein complex.
Bioenergetics and protein structure stability: Proteins fold into a specific, stable native fold, which is their lowest energy conformation. Stability is driven by a vast number of favorable non-covalent interactions, despite an entropy cost associated with ordering the polypeptide chain.
Forces stabilizing protein structure: These forces underpin the native fold:
- Hydrophobic effect: The primary driving force, where water molecules are released from the solvation layer around nonpolar groups, increasing the net entropy of the system. Predominates in the protein interior.
- Hydrogen bonds: Between backbone atoms (N-H and C=O of peptide bonds) forming secondary structures, and between side chains or side chain-backbone. Occur throughout the protein.
- Van der Waals/London dispersion forces: Weak, transient dipoles provide medium-range attractions, contributing to interior protein stability, especially in closely packed hydrophobic cores.
- Electrostatic interactions: Long-range attractions/repulsions between charged groups (ionic bonds or salt bridges), particularly stabilizing in low dielectric environments like the protein interior.
Resonance within a peptide bond: The peptide bond (C-N) exhibits partial double-bond character due to resonance stabilization between the carbonyl oxygen and the amide nitrogen. This results in the bond being rigid and planar, preventing rotation.
Rotational bonds and dihedral angles: The peptide bond itself does not rotate. Rotation is possible around the $\alpha$ -carbon, specifically at two bonds:
- Phi ( $\Phi$ ) angle: Rotation around the N– $\alpha$ C bond.
- Psi ( $\Psi$ ) angle: Rotation around the $\alpha$ C–C bond.
  These two dihedral angles dictate the conformation of the polypeptide backbone and are crucial for secondary structure formation.

Protein Secondary Structure

Ramachandran plots: These plots display the energetically favorable and unfavorable combinations of the $\Phi$ and $\Psi$ dihedral angles. They allow prediction of possible secondary structure motifs (e.g., $\alpha$ -helices, $\beta$ -sheets, left-handed helices) and identify sterically disallowed conformations.
Common secondary structures: The predominant forms are the $\alpha$ -helix and $\beta$ -sheet.
- $\alpha$ -Helix: A spiral conformation, typically right-handed, stabilized by hydrogen bonds between the carbonyl oxygen of residue $n$ and the amide hydrogen of residue $n+4$ . It contains approximately 3.6 residues per turn, with a pitch of 5.4 \unicode{x212B}. The diameter is about 1.2 \unicode{x212B} without side chains, or 2.3 \unicode{x212B} with side chains. Amino acid sequence influences stability (e.g., Proline disrupts, Glycine too flexible). Helices have a dipole moment (N-terminus positive, C-terminus negative), which can be stabilized by charged amino acids at the ends.
- $\beta$ -Sheet: Composed of multiple polypeptide strands ( $\beta$ -strands) typically in an extended conformation. These strands are stabilized by inter-strand hydrogen bonds between backbone atoms. Sheets can be parallel (N- to C-termini aligned) or antiparallel (alternating N-C and C-N).
$\beta$ -Turns: Occur when the polypeptide chain reverses direction, forming a 180 \unicode{x00B0} turn over four amino acid residues. Stabilized by a hydrogen bond between the carbonyl oxygen of residue $n$ and the amide hydrogen of residue $n+3$ . Proline (due to its cyclic structure restricting $\Phi$ angle) and Glycine (due to its conformational flexibility) are frequently found in $\beta$ -turns, especially at positions 2 and 3 respectively.
Circular dichroism (CD): This spectroscopic technique measures the differential absorption of left- and right-circularly polarized light by chiral molecules. It generates a characteristic spectrum for different secondary structures (e.g., distinct minima for $\alpha$ -helices and $\beta$ -sheets), allowing estimation of their fractional content within a protein or protein complex.

Protein Tertiary/Quaternary Structure

Tertiary structure: The full 3D arrangement of all atoms in a single polypeptide chain, stabilized by all previously mentioned non-covalent interactions (hydrophobic effect, H-bonds, van der Waals, electrostatic) and sometimes by covalent disulfide bonds. It defines the

It's completely understandable to feel overwhelmed with the amount of detailed information in these chapters. The key to active studying, especially with a large volume of content, is to break it down and engage with it rather than passively reading. Here are some strategies tailored to the structure and content of your notes:

Utilize the "Definition, Usage, and Application" Framework: Your notes explicitly outline these three perspectives for each term/concept. For every item (e.g., "consensus sequence," "Ramachandran plot," "hydrophobic effect"), actively ask yourself:
- "What is the definition of this term in my own words?"
- "How was this term used in the lecture or context of the chapter (e.g., what role does it play)?"
- "How might this term be applicable to a potential problem or question (e.g., if given a sequence, how would I identify a consensus sequence? If shown a Ramachandran plot, what can I deduce)?"
  Actively answering these for each point turns passive review into active learning.
Chunking and Prioritization: Instead of trying to learn everything at once, focus on one major section at a time. For instance, dedicate a study session to "Amino Acid Sequence and Importance," then another to "Protein Primary Sequence and Structure," and so on. Within each section, identify the absolute core concepts and terms first before diving into finer details.
Self-Quizzing and Explaining: After reviewing a section, close your notes and try to recall as much as you can. Can you list the four levels of protein structure and the forces stabilizing them? Can you explain the difference between a paralog and an ortholog? Try explaining concepts out loud to an imaginary peer or even just to yourself. This forces retrieval and highlights gaps in your understanding.
Create Concept Maps or Flowcharts: For complex processes like protein folding pathways, ligand binding dynamics, or muscle contraction, draw out flowcharts. For protein structure, a concept map connecting primary, secondary, tertiary, and quaternary levels with the stabilizing forces can be very effective.
Relate Structure to Function: A recurring theme in the notes is the relationship between protein structure and function. For every structural detail you learn (e.g., eta-turns, heme in globins), immediately ask: "How does this structural feature contribute to the protein's specific function?" This helps connect otherwise disparate pieces of information.
Flashcards (Physical or Digital): For dense terminology, creating flashcards for definitions, key properties, and clinical relevance (like for sickle cell anemia or scurvy) can be very beneficial. Test yourself frequently, explaining the term and its significance.
Focus on "Why" and "How": Don't just memorize what an oldsymbol{\alpha}-helix is; understand how it's stabilized (hydrogen bonds between n and n+4 residues) and why certain amino acids disrupt it (Proline's rigid ring, Glycine's flexibility). For experimental techniques (X-ray crystallography, NMR, Ribonuclease experiment), understand how they are performed and why they are important findings.

By actively engaging with the material using these methods, you'll find that the information becomes more digestible and less overwhelming. Good luck with your exam preparation!