Half Sphere Exposure is a protein solvent exposure

Solvent exposure

The solvent exposure of an amino acid in a protein measures to what extent theamino acid is accessible to the solvent surrounding the protein. Generally speaking, hydrophobic amino acids will be buried inside the protein and thus shielded from the solvent, while hydrophilic amino acids will be...

 measure that was first introduced by .
Like all solvent exposure measures it measures how buried amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 residues are in a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

. It is found by counting the number of amino acid neighbors within two half spheres of chosen radius around the amino acid. The calculation of HSE is found by dividing a contact number

Contact number

Contact number is a simple solvent exposure measure that measures residue burial in proteins. The definition of CN varies between authors, but is generally defined as the number of either C\beta or C\alpha atoms with in a sphere around the C\beta or C\alpha atom of the residue. The radius of the...

(CN) sphere in two halves by the plane perpendicular to the
Cβ-Cα vector. This simple division of the CN sphere results in
two strikingly different measures, HSE-up and HSE-down. HSE-up is defined as the number of Cα atoms in the
upper half (containing the pseudo-Cβ atom) and analogously HSE-down is defined as the number of Cα atoms
in the opposite sphere.

If only Cα atoms are available (as is the case for many simplified representations of protein structure), a related measure, called HSEα, can be used. HSEα uses a pseudo-Cβ instead of the real Cβ atom for its
calculation. The position of this pseudo-Cβ atom (pCβ) is derived from the positions of preceding
Cα⁻¹ and the following Cα⁺¹. The Cα-pCβ vector is calculated by adding the
Cα⁻¹-Cα⁰ and Cα⁺¹-Cα⁰ vectors.

HSE is used in predicting discontinuous B-cell epitopes. Recently Song et al. have developed an online webserver termed HSEpred to predict half-sphere exposure from protein primary sequences. HSEpred server can achieve the correlation coefficients of 0.72 and 0.68 between the predicted and observed HSE-up and HSE-down measures, respectively, when evaluated on a well-prepared non-homologous protein structure dataset. Moreover, residue contact number (CN) can also be accurately predicted by HSEpred webserver using the summation of the predicted HSE-up and HSE-down values, which has further enlarged the application of this new solvent exposure measure.