Polypeptide Calculators

0 calculators tagged with “Polypeptide”

A polypeptide is a chain of amino acids linked by peptide bonds, formed when the carboxyl group (−COOH) of one amino acid reacts with the amino group (−NH₂) of the next, releasing water (condensation reaction). Polypeptides range from short peptides (< 50 amino acids) to large proteins (hundreds to thousands of residues). The sequence of amino acids (primary structure) determines all higher levels of protein structure: secondary structure (α-helices and β-sheets from hydrogen bonding), tertiary structure (3D folding from hydrophobic interactions, disulfide bonds, ionic interactions), and quaternary structure (assembly of multiple polypeptide chains).

All Calculators

No calculators found for this topic.

Peptide Bond Formation

Condensation reaction: −COOH + H₂N− → −CO−NH− + H₂O. The peptide bond: partially double bond character due to resonance → planar (restricted rotation). N-terminus (free amino group) → C-terminus (free carboxyl group). Convention: protein sequence written N→C.

Levels of Protein Structure

  • Primary (1°): Linear sequence of amino acids (determined by mRNA/DNA); stabilized by covalent peptide bonds
  • Secondary (2°): Local regular structures; α-helix (3.6 residues/turn; backbone H-bonds i to i+4); β-sheet (parallel or antiparallel; backbone H-bonds between strands)
  • Tertiary (3°): Overall 3D fold of one polypeptide; stabilized by hydrophobic core, disulfide bonds, salt bridges, H-bonds
  • Quaternary (4°): Assembly of multiple polypeptide chains (subunits); example: hemoglobin (α₂β₂)

Protein Synthesis

Ribosomes translate mRNA 5'→3'; each codon (3 nucleotides) codes one amino acid; start codon AUG (Met); stop codons UAA, UAG, UGA. Elongation rate: ~3–10 aa/sec in eukaryotes. Co-translational folding assisted by molecular chaperones (HSP70, HSP90).

Glossary

Polypeptide
A chain of amino acids linked by peptide bonds (−CO−NH−); formed by condensation; N-terminus to C-terminus; < 50 residues = peptide; functional polypeptides with 3D structure = proteins.
Peptide Bond
−CO−NH− covalent bond between amino acid carboxyl and amino groups; partially double-bond character → planar; restricted rotation; fundamental structural unit of all proteins.
Protein Structure Levels
1° (amino acid sequence), 2° (α-helix, β-sheet from backbone H-bonds), 3° (overall 3D fold from hydrophobic core, disulfide bonds), 4° (multi-subunit assembly); each level determines function.

Frequently Asked Questions

A polypeptide is a linear chain of amino acids linked by peptide bonds. Formation: condensation reaction between the carboxyl group (−COOH) of one amino acid and the amino group (−NH₂) of the next → forms −CO−NH− peptide bond + releases H₂O. The polypeptide chain has a free amino group (N-terminus) at one end and a free carboxyl group (C-terminus) at the other. Amino acid sequence is written N→C by convention. Terminology: peptide (< 10 residues); oligopeptide (10–50); polypeptide (50+); protein (functionally active polypeptide, often with defined 3D structure). All proteins are polypeptides but not all polypeptides are functional proteins.

Primary (1°): amino acid sequence; determined by codons in mRNA; stabilized by covalent peptide bonds. Secondary (2°): local regular folding from backbone hydrogen bonds: α-helix (H-bond from C=O of residue i to N-H of residue i+4; 3.6 residues per turn; right-handed). β-sheet (H-bonds between adjacent strands; parallel or antiparallel). Random coil: non-regular regions. Tertiary (3°): overall 3D shape of one polypeptide chain; stabilized by hydrophobic interactions (core), disulfide bonds (Cys-Cys), ionic interactions, H-bonds between side chains. Quaternary (4°): association of multiple polypeptide subunits; example: hemoglobin (α₂β₂); antibodies (2 heavy + 2 light chains).

The C−N bond in the peptide group has partial double-bond character due to resonance delocalization of the lone pair on nitrogen into the carbonyl C=O: the nitrogen's p orbital overlaps with the carbonyl pi system → electrons delocalized → C−N bond has ~40% double-bond character → shorter than typical C−N single bond (~1.33 Å vs. typical 1.47 Å). Consequence: the six atoms of the peptide unit (Cα−C=O−N−H−Cα) are all coplanar (planar peptide bond). The only bond that can rotate freely is the Cα−C bond (φ, psi angles — Ramachandran plot). This planarity constrains the conformational space available to a polypeptide and is the basis for α-helix and β-sheet formation.

Polypeptide synthesis occurs on ribosomes (translation): The ribosome has three sites: A (aminoacyl), P (peptidyl), E (exit). mRNA is read 5'→3' in codons (3 nucleotides each). Each codon specifies one amino acid (via the genetic code). aminoacyl-tRNA (aa-tRNA) enters the A site; peptide bond forms by peptidyl transferase activity (catalytic RNA = ribozyme) → polypeptide transferred to A-site tRNA → ribosome translocates one codon → old A-tRNA moves to P; old P-tRNA to E. Start: AUG codon + Met-tRNA → initiates ribosome assembly. Stop: UAA, UAG, or UGA codon → release factor → polypeptide released. Rate: ~3–10 amino acids/second in eukaryotes. Co-translational folding: chaperones (HSP70, TRiC/CCT) help fold the emerging polypeptide.