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Polypeptides are continuous, unbranched polymers of amino acids joined by covalent peptide bonds. A polypeptide is the direct macromolecular product of translation and the structural precursor of every functional protein. Chains shorter than roughly 20 to 50 residues are usually called oligopeptides or simply peptides, whereas longer chains are termed polypeptides. This size boundary is a convention rather than a sharp chemical distinction.

The defining feature of a polypeptide is the amide linkage between consecutive amino acid residues, known as a peptide bond.

Every standard proteinogenic amino acid has a central alpha carbon (C_α) bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (the R-group). Proline is a structural exception because its side chain loops back to form a ring with the backbone nitrogen, making its amino group a secondary amine. At physiological pH, free amino acids exist predominantly as zwitterions, with a protonated amino group (-NH₃⁺) and a deprotonated carboxylate group (-COO⁻).

Condensation Reaction

Formally, a peptide bond forms through a dehydration (condensation) reaction in which the hydroxyl group (-OH) of one amino acid's carboxyl group and a hydrogen atom of the next amino acid's amino group are removed as a molecule of water:

In living cells, however, the reaction does not proceed by direct elimination of water from free amino acids. Each amino acid is first attached to a transfer RNA (tRNA) as an activated ester. At the ribosome, the amino group of the incoming aminoacyl-tRNA performs a nucleophilic attack on the carbonyl carbon of the ester linking the growing chain to the tRNA in the P site. This reaction is catalyzed by the peptidyl transferase center, which is composed of ribosomal RNA. The ribosome is therefore a ribozyme, and the formal loss of water is accounted for by the hydrolysis of the aminoacyl-tRNA linkage and the energy released by the activation step.

Geometry and Stability

The peptide bond has substantial partial double-bond character because of resonance between the carbonyl oxygen and the amide nitrogen. The C-N bond length is approximately 1.33 Å, intermediate between a typical C-N single bond (about 1.45 Å) and a C=N double bond (about 1.25 Å). Consequences of this resonance include:

• The six atoms of each peptide group (C_α, C, O, N, H, and the next C_α) lie in a single plane.
• Rotation about the C-N bond is strongly restricted, so the bond adopts either a trans or a cis configuration. The trans form is favored by about 1000 to 1 in most residues because it minimizes steric clashes between adjacent side chains. Peptide bonds preceding proline have a considerably higher cis population, often 5 to 10 percent.
• The backbone's conformational freedom is confined to rotations about the N-C_α bond (dihedral angle φ) and the C_α-C bond (dihedral angle ψ).

The allowed combinations of φ and ψ are visualized in a Ramachandran plot, which shows that steric hindrance excludes most of the possible conformational space. Peptide bonds are thermodynamically unstable relative to hydrolysis but kinetically very stable. In neutral water at room temperature, the half-life of an uncatalyzed peptide bond is on the order of centuries, which is why cells rely on proteases for controlled degradation.

Chain Directionality

Because each residue is joined head to tail, every polypeptide has a defined chemical polarity:

• The N-terminus (amino terminus): The start of the chain, bearing the amino group of the first residue. It is often a free amine, but may be chemically modified, for example by N-terminal acetylation or by the initiating formylmethionine in bacteria.
• The C-terminus (carboxyl terminus): The end of the chain, bearing the carboxyl group of the last residue, which may also be modified (for example, by C-terminal amidation).

Ribosomal synthesis proceeds from the N-terminus to the C-terminus. By convention, sequences are also written in this direction.

Structural vs. Functional Distinctions

The terms "polypeptide" and "protein" are often used interchangeably, but in molecular biology they emphasize different properties:

• A polypeptide describes the covalent chain of amino acids and its linear sequence.
• A protein describes the biologically functional molecule, which has a defined three-dimensional structure (or, in some cases, a functional disordered ensemble) and may include non-peptide components.

Subunit Assembly and Intrinsically Disordered Regions

A functional protein may consist of a single folded polypeptide, as in myoglobin, a 153-residue chain that binds one heme group. Alternatively, it may assemble from several polypeptide chains, as in adult hemoglobin, which contains two alpha-globin and two beta-globin polypeptides, each carrying its own heme. Many polypeptides are also chemically altered after synthesis by post-translational modifications such as phosphorylation, glycosylation, and proteolytic cleavage, which can be essential for activity.

Not every polypeptide forms a stable fold. Intrinsically disordered proteins and disordered regions lack a single fixed structure under physiological conditions yet perform important functions in signaling and regulation.

Combinatorial Sequence Diversity

Because each position in a chain can be occupied by any of 20 standard amino acids, the number of possible sequences for a polypeptide of length n is:

A chain of only 100 residues therefore has 20¹⁰⁰ (about 10¹³⁰) possible sequences, vastly more than the number of sequences that have ever been realized in biology.

Molecular Mass Estimation

The average mass of an amino acid residue (after loss of water) is approximately 110 daltons, so the molecular mass of a polypeptide can be estimated as:

The transformation of a linear polypeptide into an active enzyme or structural protein is described in terms of four levels of structural organization:

1. Primary structure: The exact linear sequence of amino acids, specified by the nucleotide sequence of the mRNA according to the genetic code.
2. Secondary structure: Local folding patterns stabilized by hydrogen bonds between the backbone carbonyl oxygen of one residue and the backbone amide hydrogen of another. The two most common motifs are the alpha helix, a right-handed coil with 3.6 residues per turn in which each carbonyl bonds to the amide four residues ahead, and the beta sheet, in which extended strands align side by side in parallel or antiparallel arrangements.
3. Tertiary structure: The complete three-dimensional fold of a single polypeptide, stabilized by interactions among side chains and the backbone. These include the hydrophobic effect (the burial of non-polar side chains away from water, which is driven largely by the entropy of the solvent), salt bridges, hydrogen bonds, van der Waals forces, and covalent disulfide bonds between cysteine residues. Disulfide bonds form mainly in oxidizing environments such as the endoplasmic reticulum and the extracellular space.
4. Quaternary structure: The arrangement of two or more polypeptide chains (subunits) into one functional complex. Subunits are held together mostly by non-covalent interactions, although disulfide bonds can also link them.

Determinants of Folding

The classic experiments of Christian Anfinsen on ribonuclease showed that, for many small proteins, the amino acid sequence contains the information needed to specify the native structure under suitable conditions. This principle is known as Anfinsen's dogma. It is, however, an incomplete description of folding in cells. The chemical properties of the side chains (charge, polarity, and size) are the main determinants of the native fold, but the cellular environment, post-translational modifications, and assistance from other proteins also play important roles.

Folding Kinetics and Chaperones

A polypeptide cannot find its native structure by random search. Levinthal's paradox points out that if each residue had only three accessible conformations, a chain of n residues would have 3ⁿ possible conformations, a number so large that sequential sampling would take longer than the age of the universe even for modest chain lengths. Real proteins fold in milliseconds to seconds because folding is directed down a funnel-shaped energy landscape in which partially formed native-like contacts are progressively stabilized.

In cells, folding is further assisted by molecular chaperones such as the Hsp70 and Hsp60 families, which prevent premature aggregation of hydrophobic regions and give misfolded chains additional chances to refold. Polypeptides that fail to reach their native structure may form aggregates, and protein misfolding is associated with diseases such as Alzheimer's disease and the prion diseases.

How does the Ramachandran plot restrict polypeptide backbone conformations?

Because the peptide bond itself is planar and rigid (ω ≈ 180°), a polypeptide's backbone flexibility is governed by the torsion angles φ (around the N–C_α bond) and ψ (around the C_α–C bond). The Ramachandran plot maps which (φ, ψ) combinations avoid steric collisions between main-chain atoms and C_β side-chain atoms, confining >90% of non-glycine residues to right-handed α-helical and β-sheet regions.

Why are glycine and proline outliers on a Ramachandran plot?

Glycine lacks a β-carbon side chain (its R-group is a single hydrogen atom), eliminating steric clashes and allowing it to occupy broad conformational regions forbidden to other amino acids, such as sharp turns and collagen triple helices. Conversely, proline's cyclic pyrrolidine side chain covalently locks its φ angle near −60° ± 15°, making it a rigid helix breaker.

How does Levinthal's paradox demonstrate the existence of a folding energy funnel?

Cyrus Levinthal noted that a 100-residue polypeptide with 3 conformations per residue has 3¹⁰⁰ (≈ 5 × 10⁴⁷) states; sampling each at picosecond rates (10⁻¹² s) via random walk would require ~10²⁸ years. Because polypeptides actually fold in milliseconds, folding cannot be a random search—it proceeds down a biased thermodynamic energy funnel where cooperative local hydrophobic collapse rapidly narrows conformational space.