SpAB protein domain
# SpAB protein domain

In molecular biology, the domain B, refers to the immunoglobulin-binding domain found in the Staphylococcus aureus virulence factor protein A (SpA). Hence, it is abbreviated to $SpA_B.$

## Function

$SpA_B$ enables theStaphylococcus aureus bacteria to evade the host's immune system through the disruption of opsonization and phagocytosis. It does this though $SpA_B$ binding to the Fc fragment of IgG.

## Structure

The B domain of SpA (SpAB) consists of three a-helices which are retained upon interaction with the Fc fragment of IgG. Protein A contains five highly homologous immunoglobulin (Ig)-binding domains in tandem (designated domains E, D, A, B and C), which share a common structure consisting of three helices in a closed left-handed twist. Protein A can exist in both secreted and membrane-bound forms, and has two distinct Ig-binding activities: each domain can bind Fc-gamma (the constant region of IgG involved in effector functions) and Fab (the Ig fragment responsible for antigen recognition).

The native state of the B domain, deviates a lot since its inter-helical angles fluctuate. It appears to be relatively thermodynamically more stable than the E domain. The increased stability of the B domain may be due to heightened mobility, and therefore entropy, in the native state and decreased mobility entropy in the more compact denatured state.

