Cysteinepoor

Cysteinepoor, or cysteine-poor, is a descriptive term used in biochemistry and proteomics to refer to proteins or protein regions that have an unusually low number of cysteine residues relative to the protein’s length or to the organism’s typical cysteine content. Cysteine content is one factor that influences protein folding, stability, and redox sensitivity because cysteine side chains can form disulfide bonds and participate in catalytic and regulatory motifs.

Cysteine residues contribute thiol groups that readily form disulfide bonds, stabilizing extracellular and secreted proteins and enabling redox-based regulation. Consequently, many proteins that function in oxidizing environments or that are exported from the cell tend to be cysteine-rich. In contrast, cysteine-poor proteins are commonly found in reducing cellular compartments such as the cytosol and mitochondrial matrix, where disulfide bonds are less favored, or among protein families that do not rely on extensive disulfide cross-links for stability.

Quantification of cysteine content is typically expressed as a percentage of residues that are cysteine or as cysteines per 100 amino acids. Thresholds for labeling a protein as cysteine-poor vary by organism and context. In annotation and comparative proteomics, cysteine-poor regions may be noted as indicative of reduced disulfide-bonding potential, redox-insensitive structure, or alternative stabilizing interactions such as hydrophobic packing and salt bridges.

Examples are broadly observed among intracellular proteins and some structural components, where low cysteine content aligns with a reducing environment and different evolutionary constraints. See also: cysteine-rich proteins, disulfide bond, redox biology.