Table 1.

Highly ranked residues that discriminate the pH1N1 strain from circulating human H1N1 strains in the hemagglutinin RBD

Residue number in structure 3lzg	Rank*	Circulating human strain characteristics†	pH1N1 characteristics‡	Structural comments
145	1	S	K	In antigenic site Ca. A change from the (positively charged) K to (polar) S can change the stereochemistry of the antigenic site and affect receptor specificity.
171	2	N	D	In antigenic site Ca. May change the physicochemical characteristics of Ca.
225	3	—	—	In antigenic site Ca. Known specificity determinant for altering avian to human H1N1 sequences.
219	4	E	I	Close to antigenic site Sb. The E-to-I mutation may affect the antigenic site’s structure.
261	5	S	E	Residues 261 + 263 + 264 form a cluster in the structure. Residue 261 is very close to antigenic site Ca. From the other side, residues 263 and 264 are close to antigenic site Cb. These positions may be a continuous part of the antigenic sites. The alterations may affect the protein structure or the approximate antigenic sites.
132	6	I	P,S	In the vicinity of the Sa antigenic site.
206	7	—	—	In antigenic site Ca.
173	8	—	G	In antigenic site Ca. The mutation may alter the antibody binding.
264	9	F	A	See comments for position 261.
263	10	G	N

Positions are numbered as in the A/California/04/2009 H1N1 strain (PDB ID code 3lzg) structure.

^*Rank of contribution to discrimination given to the detected position (see Computational Methods).

^†The amino acid suggested to characterize circulating human H1N1 strains by the algorithm.

^‡The amino acid suggested to characterize the pH1N1 strain. The symbol “—” indicates that no characteristic amino acid was identified.