If I have the protein sequence for a particular gene product, can an siRNA be designed to target this protein?

siRNAs function at the mRNA level, not at the protein level. In order to design an siRNA, the precise target mRNA nucleotide sequence is required. Due to the degenerate nature of the genetic code, it is impossible to accurately predict the correct nucleotide sequence from the peptide sequence. Additionally, since the function of siRNAs is to cleave mRNA sequences, it is important to use the mRNA nucleotide sequence and not the genomic sequence for siRNA design. Pre-mRNA processing removes intron sequences that are contained in the genomic sequence. Designs using genomic information might inadvertently target introns and as a result the siRNA would not be functional for silencing the corresponding mRNA.