what does Pro→Glu substitution mean?
You are looking at a protein here, which is mutated in the higher host specificity, so glucose is not involved here. What is written here are the amino acids in the three-letter-code, the notation Pro→Glu means that Proline (Pro) is replaced by Glutamic acid (Glu).

The notation Pro16 means the proline on amino acid position 16 in the protein. You often find this replacements noted as Pro16Glu or P16E (in the 1-letter-code)