Contact order

A normalized measure of how far apart along a protein sequence the residues forming native-state contacts tend to be.

Version

v2 · 2026-09-19 · History

Domain-specific #

3874

Domain group

Natural Sciences

Origin domain

Biology & Ecology

Subdomain

Structural Biology → Biology & Ecology

Core Idea

Relative contact order averages sequence separation over declared native contacts and divides by chain length; contact definition, domain boundary and structural ensemble materially affect the value. A native structure supplies contacting residue pairs, their sequence separations are summed and normalized by contact count and protein length to express the locality of the fold topology. The abstraction is therefore identified by a declared carrier, a transformation or constraint over that carrier, and an invariant that tells an analyst whether the named structure is genuinely present.

How would you explain it like I'm…

How Far Apart Touching Beads Are

A protein is like a long string of beads that folds up into a ball. When it's folded, some beads end up touching each other. Contact order asks: are the touching beads usually close together on the string, or far apart? If lots of far-apart beads touch, the contact order is high.

How Far Apart Touching Parts Are

A protein is a long chain of small parts that folds into a shape. In the folded shape, some parts touch each other. Contact order measures, for all the touching pairs, how far apart the two parts are along the chain, on average. Then it divides by the length of the chain so short and long proteins can be compared. A low number means most touches are between nearby parts; a high number means many touches link parts far apart on the chain.

Protein Fold Locality Measure

Contact order describes how 'local' a protein's folded structure is. Starting from the native, folded structure, you list the pairs of amino acid residues that are in contact, using a distance cutoff. For each pair you find the sequence separation, the number of positions between them along the chain. Relative contact order averages these separations over all contacts and then divides by the chain's length, so proteins of different sizes can be compared. The result depends a lot on choices: how close counts as a contact, where the protein's domain boundaries are drawn, and which structure or set of structures is used. It is sometimes compared with how fast proteins fold, but any such comparison needs these choices stated.

 

Relative contact order quantifies the locality of a protein's native fold topology. From a native structure, one identifies contacting residue pairs according to declared distance and sequence-separation thresholds, sums their sequence separations, and normalizes by the number of contacts and by chain length. Low values indicate folds dominated by contacts between residues near each other in sequence, high values folds with many long-range contacts. The value is materially affected by the contact definition, the domain boundaries, the residue-index convention and whether a single structure or an ensemble is used. A properly stated contact order therefore specifies the protein chain and length, the native structure or ensemble, the thresholds, the included pairs, the normalization formula and uncertainty, along with the basis of any comparison to folding rates.

Scope of Application

Contact order belongs to structural biology and is useful where the analyst can specify the typed structural biology carrier, including objects, relations, parameters, conventions, evidence, boundaries, and comparison targets, then evaluate the protein chain and length, native structure and ensemble, residue-index convention, contact-distance and sequence-separation thresholds, included contact pairs, normalization formula, uncertainty and any folding-rate comparison are explicit. The scope is broad within that domain but bounded by the need for the protein chain and length, native structure and ensemble, residue-index convention, contact-distance and sequence-separation thresholds, included contact pairs, normalization formula, uncertainty and any folding-rate comparison are explicit. Descriptive structural-biology metric only; no wet-lab, diagnostic, or intervention procedure is provided.

Clarity

The abstraction clarifies a crowded vocabulary by making the protein chain and length, native structure and ensemble, residue-index convention, contact-distance and sequence-separation thresholds, included contact pairs, normalization formula, uncertainty and any folding-rate comparison are explicit the center of the account. A claim should name the carrier, the governing operation or relation, the applicable assumptions, and the recognition test. A bare label is insufficient because the name Contact order can be used for a formal identity, an implementation, or a neighboring result unless carrier and convention are stated.

Manages Complexity

Without the abstraction, an analyst must reason directly over many local details: the carrier roles, admissibility assumptions, competing conventions, derived invariants, boundary cases, and proof or validation obligations specific to Contact order. Contact order compresses them into the roles in the structural signature. That compression permits comparison across instances without erasing the variables that determine validity. It also exposes which details may be varied safely and which are constitutive.

Abstract Reasoning

1. Identify the carrier. State what the elements, states, objects, or observations are: the typed structural biology carrier, including objects, relations, parameters, conventions, evidence, boundaries, and comparison targets. Reject examples whose alleged carrier belongs to a different problem.
2. Lock the constitutive rule. Express the protein chain and length, native structure and ensemble, residue-index convention, contact-distance and sequence-separation thresholds, included contact pairs, normalization formula, uncertainty and any folding-rate comparison are explicit independently of one notation or implementation.

Knowledge Transfer

Knowledge transfers strongly among subfields of structural biology because they reuse the typed structural biology carrier, including objects, relations, parameters, conventions, evidence, boundaries, and comparison targets, A native structure supplies contacting residue pairs, their sequence separations are summed and normalized by contact count and protein length to express the locality of the fold topology., and type the carrier, state every parameter and convention in the definition, test that the protein chain and length, native structure and ensemble, residue-index convention, contact-distance and sequence-separation thresholds, included contact pairs, normalization formula, uncertainty and any folding-rate comparison are explicit, compare the nearest accepted identity, and report counterexamples, uncertainty, and limiting cases.

Relationships to Other Abstractions

Current abstraction Contact order Domain-specific

Parents (1) — outgoing typed relations

• Contact order is a kind of Metric Prime

  It remains its own entry because its identity is fixed by the protein chain and length, native structure and ensemble, residue-index convention, contact-distance and sequence-separation thresholds, included contact pairs, normalization formula, uncertainty and any folding-rate comparison.

Hierarchy path (1) — routes to 1 parentless root

• Contact order → is a kind of Metric → is a kind of Function (Mapping)

Neighborhood in Abstraction Space

Contact order sits in a crowded region of the domain-specific corpus (31^st percentile for distinctiveness): several abstractions share nearly its structure, so a description that fits it tends to fit its neighbors too.

Family — Molecular & Cellular Biology Mechanisms (16 abstractions)

Nearest neighbors

• Single-strand conformation polymorphism — 0.89
• Ka/Ks ratio — 0.89
• Genetic code — 0.89
• RAN translation — 0.89
• Protein quinary structure — 0.89

Computed from structural-signature embeddings · 2026-10-25