# Protein Blocks

Protein Blocks (PBs) are structural prototypes defined by de Brevern et al in 2000 [1]. The 3-dimensional local structure of a protein backbone can be modelized as an 1-dimensional sequence of PBs. In principle, any conformation of any amino acid could be represented by one of the sixteen available Protein Blocks.

PBs are labeled from a to p (see Figure 1): the PBs m and d can be roughly described as prototypes for alpha-helix and central beta-strand, respectively. PBs a to c primarily represent beta-strand N-caps and PBs e and f, beta-strand C-caps; PBs a to j are specific to coils, PBs k and l to alpha-helix N-caps, and PBs n to p to alpha-helix C-caps.

Figure 1

Schematic representation of the sixteen protein blocks, labeled from a to p (Creative commons CC-BY).

Figure 2

3D representation of the barstar protein (PDB ID 1AY7, chain B) (Creative commons CC-BY).

For instance, the 3D structure of the barstar protein represented in Figure 2 can be translated in a 1D-sequence of Protein Blocks:

    ZZdddfklpcbfklmmmmmmmmnopafklgoiaklmmmmmmmmpacddddddehkllmmmmnnommmmmmmmmmmmmmnopacddddZZ

The conformations of the 89 residues of the barstar are translated into a sequence of 89 protein blocks. Note that “Z” corresponds to amino acids for which a protein block cannot be assigned. As a matter of fact, the assignment of a given residue n requires is based on the conformations of residues n-1, n-1, n, n+1 and n+2. Therefore, a protein block cannot be assigned to the two first (N-termini) and two last (C-termini) residues of a polypeptide chain.

## Neq

Neq is calculated as follows:

Figure 3

Neq formula

Where fx is the probability of PB x. Neq quantifies the average number of PBs at a given position in the protein sequence. A Neq value of 1 indicates that only one type of PB is observed, while a value of 16 is equivalent to a random distribution.

[1] | de Brevern, C. Etchebest, and S. Hazout. Bayesian Probabilistic Approach for Predicting Backbone Structures in Terms of Protein Blocks. Proteins 41:271-87 (2000).