2 Interactions between proteins
37 important questions on 2 Interactions between proteins
What factors influence the functionality of proteins, particularly their solubility and other properties?
- Interactions between proteins and other components.
- Non-covalent molecular interactions:
- - Van der Waals
- - Hydrogen bonds
- - Hydrophobic
- - Electrostatic
- One covalent interaction: disulphide bridge.
What are the learning goals related to protein properties in this course?
- Understand surface charge density.
- Understand surface hydrophobicity.
- Analyze the impact of pH and salts on electrostatic interactions between proteins.
What are protein interactions and how are they influenced by surface properties?
- Protein interactions involve attraction and repulsion.
- Attraction: Two hydrophobic particles attract.
- Repulsion: Two positively charged particles repel.
- Properties influencing interactions:
- - Surface charge density affects electrostatic interactions.
- - Surface hydrophobicity affects hydrophobic interactions.
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Describe surface hydrophobicity as illustrated in Figure 9.
- Surface hydrophobicity involves regions attracting water or not.
- Hydrophilic areas are shown in blue.
- Hydrophobic areas are shown in yellow.
- Surface properties affect protein interactions in solutions.
How does surface charge density affect protein interactions?
- Surface charge density refers to distribution of electrical charge.
- Negative charge: attracts positive charges.
- Positive charge: attracts negative charges.
- Changes with pH, affecting protein function and interactions.
What variations in surface charge density occur at different pH levels?
- Surface charge density changes with pH.
- α-chymotrypsin:
- - pH 5: predominantly neutral/positive.
- - pH 9: shows increased negative charge.
- Lysozyme:
- - pH 5: positive charge dominant.
- - pH 11: more neutral/negative areas.
What are the color codes used to represent surface areas in globular water-soluble proteins and A-helical membrane proteins?
- Blue indicates hydrophilic surfaces
- Orange denotes hydrophobic areas
- Dark salmon represents residues that are in between
How are the surface representations of α-chymotrypsin and lysozyme colored according to electrostatic potential at different pH levels?
- At pH = 5:
- - -5 keV is colored red
- - +5 keV is colored blue
- pI: α-chymotrypsin is at pH 9, lysozyme at pH 11
What is surface charge density in proteins?
- Influences ZETA POTENTIAL (mV)
- Depends on pH conditions
- Changes at pH equal to pI
How does the zeta potential relate to protein surface charges?
- Equal charges at pH = pI
- Example: pH 9 for α-chymotrypsin
- Negative zeta potential at pH below pI
What factors influence the surface hydrophobicity of proteins?
- Presence of polar groups (hydrophilic)
- Presence of non-polar groups (hydrophobic)
- Protein folding and denaturation
What happens to hydrophobicity during protein unfolding?
- Non-polar groups become exposed
- Regional changes due to folding alterations
- Important for globular proteins
How does protein folding affect surface charge density and hydrophobicity?
- Charge distribution affecting surface charge density
- Arrangement of polar and non-polar groups
- Changes upon denaturation impacting both properties
How do pH and salts affect electrostatic interactions between proteins in solution?
- Proteins in solution have a water layer that includes ions.
- This layer moves with the protein.
- The zeta potential is measured at this layer's edge.
- Changes in pH affect protein charge.
- Salts can shield or enhance electrostatic interactions.
How does the pH of the solution influence the charge of proteins?
- Net positive charge
- Net negative charge
- Net zero charge
- Charge impacts repulsion at distance.
What happens to the repulsive energy of particles at large distances?
- Particles don't feel repulsive energy
- Minimal charge equals minimal repulsion
- Distance impacts interaction potential.
What influence do charge and counterion concentration have on repulsive energy?
- The charge of the protein
- The concentration of counterions
What is illustrated in the graph regarding repulsive energy between particles?
- Interaction potential as a function of distance
- Different net charges from -25 to almost zero
- Charge change alters repulsion at all distances.
What is the outcome when the charge on the protein is altered?
- Variation in repulsion at all distances
- Starts from the wall of the particle
- Distance directly influences interaction.
What does the electrostatic repulsive interaction energy between two particles depend on according to the graph?
- Interaction energy decreases with increasing distance.
- Higher net charges (-25 to -1) result in stronger repulsive energy at the same distance.
- At ~1 nm, repulsion is significantly higher for high-net charges.
How does pH affect the interaction between proteins?
- Protein charge varies with pH.
- At pH equal to pI, proteins have minimal repulsion.
- When pH > pI, proteins become negatively charged.
- High repulsion occurs with strong negative charge.
What do the graphs of electrostatic repulsion look like for positively charged particles ranging from +1 to +18?
- Electrostatic repulsion increases with charge
- Moderate attraction at lower charges
- Transition to stronger repulsion as charge approaches +18
- Graphs shift positively compared to negative interactions
How do counterions influence the interaction potential between charged proteins?
- Screening the protein's charge
- Reducing effective repulsion
- Introducing ions such as Na+ and Cl-
- Enhancing stability in food products through ions like Ca2+
What principle is illustrated by the surrounding of a negatively charged protein in a NaCl solution?
- Screening of charges by counterions: The negatively charged protein attracts positively charged sodium ions.
- This attraction leads to further attraction of negatively charged chloride ions.
- Zeta potential is measured at the grey dashed line where the protein appears net-neutral.
- Counterions create a water layer, making the protein seem neutral at a distance.
What is the effect of salt on electrostatic repulsion between negatively charged proteins?
- Interaction potential at 0 distance = 25
- Higher salt allows closer protein proximity
- Charges are screened by salt
How is ionic strength (I) calculated in protein studies?
- I = 0.5 Σ C i z i²
- C i represents molar concentration of ions
- z i refers to ion valence
Why should pH be measured when using protein ingredients?
- Proteins produced at specific pH
- Presence of basic and acidic groups
- Potential changes in pH during dissolution
- Importance of buffers
Why is it important to re-measure pH at higher protein concentrations when dissolving?
- Changes in pH from dissolved proteins
- Basic and acidic groups acting as buffers
- Ensuring stable protein properties
What are the molar salt concentrations for the samples made with different salt concentrations (0.100 g/L, 0.200 g/L, 0.500 g/L)?
- 0.100 g/L: 0.0043 mol/L NaCl
- 0.200 g/L: 0.0086 mol/L NaCl
- 0.500 g/L: 0.0215 mol/L NaCl
Are the salt concentrations of 0.100 g/L, 0.200 g/L, and 0.500 g/L logical for studying the effect of salt on protein solubility?
- They are relatively low.
- Higher concentrations may be needed to effectively study protein interactions.
- Increased salt can strongly screen protein charges.
What concentration is needed to strongly screen the charges on proteins and allow them to approach each other closely?
- Effectively screen charges.
- Allow proteins to approach each other closely.
- Usually higher than 0.500 g/L for significant effects.
How is the interactional potential (U) defined in relation to proteins?
- Charge of the protein (Z).
- Separation distance (R).
- Debye length (κ) based on salt concentration.
What is the significance of the Stern layer in the context of protein surface potential?
- Ions tightly bound to the protein surface.
- Thickness equal to the radius of counterions.
- Determines Stern potential, which is lower than surface potential.
What is measured to determine the zeta (ζ)-potential of a particle in solution?
- Measure particle mobility in an electric field.
- Analyze the layer of water and counter ions around the particle.
- Calculate zeta-potential, which is lower than Stern potential.
How does the size of a protein affect its surface charge density?
- Smaller proteins (1 nm radius) have higher surface charge density.
- Larger proteins (3 nm radius) with the same charge have lower density.
- Surface charge density affects protein interactions.
What challenges are associated with measuring the surface potential of proteins?
- Presence of counter ions in solution.
- Difficulty in determining the actual potential near the protein surface.
- The relationship between surface, Stern, and zeta potentials complicates measurements.
What factors can influence the Debye length (κ) in relation to protein interactions?
- Salt concentration in solution.
- Ion types present.
- Temperature of the solution which affects ionic strength.
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