Nonselenoprotein

Nonselenoprotein refers to any protein that does not contain selenocysteine, the 21st amino acid. Selenocysteine is encoded by a UGA codon in the presence of a SECIS element and specialized translational factors, allowing the ribosome to insert Sec rather than terminate translation. Nonselenoproteins are translated using the universal genetic code without Sec recoding, so they do not carry selenocysteine residues.

Most proteins are nonselenoproteins. The term is commonly used in comparative genomics, evolutionary biology, and biochemistry to distinguish proteins that require the Sec decoding system from those that do not. The distribution of selenoproteins varies across species: some lineages retain a Sec insertion apparatus and many selenoproteins, while others have lost the machinery and rely exclusively on nonselenoproteins. In humans, a relatively small set of genes encode selenoproteins, whereas the bulk of the proteome is nonselenoprotein.

In organisms that use Sec, selenocysteine often contributes catalytic advantages or redox properties, but many essential reactions are carried out by cysteine-containing enzymes or other residues in nonselenoproteins. Where Sec is absent, cysteine-containing homologs can often substitute at Sec positions; these substitutions are a focus of research into redox biology and enzyme evolution.

Nonselenoproteins thus represent the majority of expressed proteins in most organisms, reflecting the broader presence of cysteine- and other residue-based redox chemistry in biology.