Secondary struture + protein stability

What is protein conformation

The three-dimensional arrangement of atoms in a protein that can change without breaking covalent bonds.

What is the native conformation

The folded structural state associated with a protein's normal biological function under specified conditions.

What is protein stability

The tendency of a protein to maintain its folded state rather than unfold or adopt alternative conformations.

What is the relationship between protein sequence, structure, and function

The amino acid sequence influences the protein's structure, which in turn determines its function.

What interactions stabilize protein structure

Hydrogen bonds, ionic interactions, van der Waals interactions, hydrophobic interactions, and, where present, covalent disulfide bonds.

What is a hydrogen bond in a protein

A noncovalent interaction involving a hydrogen attached to an electronegative atom and another suitable electronegative atom.

What is an ionic interaction in a protein

An electrostatic attraction between oppositely charged groups.

What is a van der Waals interaction

A weak interaction arising from attractions between atoms due to their instantaneous or induced dipoles.

What is a hydrophobic interaction

The tendency of nonpolar groups to cluster away from water, contributing to protein folding in aqueous environments.

Why can numerous weak interactions strongly stabilize a protein

Their combined energetic contributions can be substantial even though each individual interaction is relatively weak.

What are two general principles of soluble protein structure

Maximize favorable hydrogen-bonding and ionic interactions, and minimize the exposure of hydrophobic groups to water.

Why can burying an unsatisfied polar group destabilize a protein

The polar group loses favorable interactions with water without gaining sufficient compensating interactions inside the protein.

What is a disulfide bond

A covalent bond formed by oxidation of the thiol groups of two cysteine residues.

Where are disulfide bonds commonly found

They are common in extracellular and secreted proteins, although their occurrence depends on the cellular environment.

Why is the peptide bond rigid and planar

Resonance gives the C–N bond partial double-bond character, restricting rotation.

What is the peptide group

The approximately planar group of six atoms associated with a peptide bond: Cα–C(=O)–N–H–Cα across two adjacent residues.

What is the omega ((\omega)) angle

The dihedral angle around the peptide C–N bond, which is usually in the trans configuration near 180°.

What is the phi ((\phi)) angle

The backbone dihedral angle around the N–Cα bond.

What is the psi ((\psi)) angle

The backbone dihedral angle around the Cα–C(carbonyl) bond.

Why are many phi and psi angle combinations prohibited

Steric clashes occur when backbone or side-chain atoms occupy incompatible positions.

What is a Newman projection used for in protein structure

It represents a view along a bond to visualize the relative orientation of atoms and dihedral angles.

What is a Ramachandran plot

A plot of phi and psi angles showing which backbone conformations are sterically allowed or favored.

Why does glycine have a distinctive Ramachandran plot

Its small side chain allows a wider range of backbone conformations.

Why does proline have a distinctive Ramachandran plot

Its cyclic side chain restricts backbone rotation and limits the range of permitted conformations.

How is a Ramachandran plot used to validate a protein structure

The observed backbone angles are checked against allowed regions to identify unusual conformations that may indicate structural problems.

What is protein secondary structure

Regular local conformations of the polypeptide backbone stabilized by characteristic hydrogen-bonding patterns.

What are the main regular secondary structures

α-helices and β-sheets.

What are examples of irregular secondary structures

Loops and β-turns.

Do side chains form the defining hydrogen bonds of an α-helix or β-sheet

No. The defining hydrogen bonds mainly involve backbone carbonyl oxygen and amide hydrogen atoms, although side chains affect stability.

What is an α-helix

A coiled protein backbone structure stabilized by regularly spaced intrachain hydrogen bonds.

What is the handedness of a typical protein α-helix

Right-handed.

How many amino acid residues occur per turn of a typical α-helix

Approximately 3.6 residues per turn.

What is the rise per turn of an α-helix

Approximately 5.4 Å per turn.

What is the hydrogen-bonding pattern in an α-helix

The carbonyl oxygen of residue (i) hydrogen-bonds to the amide hydrogen of residue (i+4).

Where do α-helical side chains project

Outward from the helical backbone.

What is the helix dipole

A net dipole created by the alignment of individual peptide-bond dipoles along an α-helix.

Which charges are generally favored at the ends of an α-helix because of its dipole

Negative residues near the positively charged N-terminal end are unfavorable; more generally, negative charges are favored near the positive N-terminal end and positive charges near the negative C-terminal end.

How can amino acid side chains affect α-helix stability

Their size, charge, polarity, and interactions with neighboring residues can favor or disrupt helical structure.

What is a β-sheet

A secondary structure formed by extended polypeptide strands connected by backbone hydrogen bonds.

Where do β-sheet side chains project

Alternately above and below the plane of the sheet.

What is a parallel β-sheet

A sheet in which neighboring strands run in the same N-to-C direction.

What is an antiparallel β-sheet

A sheet in which neighboring strands run in opposite N-to-C directions.

What is the key difference between parallel and antiparallel β-sheet hydrogen bonding

Their strand orientations produce different hydrogen-bonding geometries; antiparallel hydrogen bonds are generally more linear.

Why can β-sheets form between separate polypeptide chains

Backbone hydrogen bonds can connect strands belonging to different chains.

Why can β-sheets contribute to protein aggregation

Exposed β-strands can form intermolecular hydrogen bonds with strands from other protein molecules.

What is a β-turn

A short segment of backbone that reverses chain direction, often connecting adjacent antiparallel β-strands.

Why are glycine and proline common in β-turns

Glycine permits tight turns because of its flexibility, while proline's restricted geometry can favor certain turn conformations.

What is circular dichroism (CD) spectroscopy

An experimental technique that measures differential absorption of circularly polarized light and can be used to estimate protein secondary structure.

How can CD spectroscopy estimate protein secondary structure

Spectral patterns are analyzed to estimate the proportions of α-helices, β-sheets, and other conformations.

How can CD spectroscopy be used to study protein denaturation

Changes in the spectrum can reveal loss or alteration of secondary structure as a protein unfolds.