HCXXGXXR

HCXXGXXR is a short amino acid sequence motif described in protein sequence analysis as eight residues long, written using one-letter codes as H-C-X-X-G-X-X-R. Here, X denotes any amino acid. The motif appears across diverse protein families in bacteria, archaea, and eukaryotes, often located in regions associated with catalysis or substrate binding. Because it is compact and contains residues capable of participating in chemical reactions, HCXXGXXR is frequently used in motif-based annotation and screening to identify candidate enzymes or functional sites.

Functional interpretation of the motif is highly context-dependent. In some proteins, the histidine and cysteine may participate directly in catalytic activity or metal coordination; glycine can confer conformational flexibility in the active site; arginine may help stabilize substrates or transition states through electrostatic interactions. However, the presence of HCXXGXXR is not diagnostic of a single activity, and many proteins lacking a related function may coincidentally contain the motif.

Detection and research: HCXXGXXR is identified through sequence alignment and motif-search methods, as well as model-based annotation in bioinformatics pipelines. Experimental validation is typically required to establish the exact role of the motif in a given protein, using approaches such as site-directed mutagenesis, enzymatic assays, or structural analysis.

See also: protein motifs, active-site patterns, sequence annotation. Limitations: motif-based predictions are hypotheses and should be confirmed with empirical evidence.