A method for recombinantly producing a high molecular weight heparosan polymer, the method comprising the steps of: culturing a recombinant host cell containing a nucleotide sequence encoding a polypeptide having heparosan synthase activity under conditions appropriate for the expression of the heparosan synthase, wherein at least one of: (a) the polypeptide having heparosan synthase activity is at least 90% identical to at least one of SEQ ID NOS:2, 4, and 6-8, and the nucleotide sequence encoding the polypeptide has been gene-optimized for expression in the recombinant host cell; (b) the polypeptide having heparosan synthase activity has 1-20 amino acid additions, deletions, and/or substitutions when compared to at least one of SEQ ID NOS:2, 4, and 6-8, and the nucleotide sequence encoding the polypeptide has been gene- optimized for expression in the recombinant host cell; (c) the polypeptide is encoded by the nucleotide sequence of at least one of SEQ ID NOS:9-11; (d) the polypeptide is encoded by a nucleotide sequence that is at least 90% identical to at least one of SEQ ID NOS:9-11; and (e) the nucleotide sequence encodes a Pasteurella heparosan synthase; and isolating heparosan polymer produced by the heparosan synthase, wherein the isolated heparosan polymer is biocompatible with a mammalian patient and biologically inert within extracellular compartments of a mammalian patient, and wherein the isolated heparosan polymer is represented by the structure (-GlcUA-beta1,4-GlcNAc-alpha-1,4-) n, wherein n is a positive integer greater than or equal to 2,000. culturing a recombinant host cell containing a nucleotide sequence encoding a polypeptide having heparosan synthase activity under conditions appropriate for the expression of the heparosan synthase, wherein at least one of: (a) the polypeptide having heparosan synthase activity is at least 90% identical to at least one of SEQ ID NOS:2, 4, and 6-8, and the nucleotide sequence encoding the polypeptide has been gene-optimized for expression in the recombinant host cell; (b) the polypeptide having heparosan synthase activity has 1-20 amino acid additions, deletions, and/or substitutions when compared to at least one of SEQ ID NOS:2, 4, and 6-8, and the nucleotide sequence encoding the polypeptide has been gene- optimized for expression in the recombinant host cell; (c) the polypeptide is encoded by the nucleotide sequence of at least one of SEQ ID NOS:9-11; (d) the polypeptide is encoded by a nucleotide sequence that is at least 90% identical to at least one of SEQ ID NOS:9-11; and (e) the nucleotide sequence encodes a Pasteurella heparosan synthase; and (a) the polypeptide having heparosan synthase activity is at least 90% identical to at least one of SEQ ID NOS:2, 4, and 6-8, and the nucleotide sequence encoding the polypeptide has been gene-optimized for expression in the recombinant host cell; (b) the polypeptide having heparosan synthase activity has 1-20 amino acid additions, deletions, and/or substitutions when compared to at least one of SEQ ID NOS:2, 4, and 6-8, and the nucleotide sequence encoding the polypeptide has been gene- optimized for expression in the recombinant host cell; (c) the polypeptide is encoded by the nucleotide sequence of at least one of SEQ ID NOS:9-11; (d) the polypeptide is encoded by a nucleotide sequence that is at least 90% identical to at least one of SEQ ID NOS:9-11; and (e) the nucleotide sequence encodes a Pasteurella heparosan synthase; and isolating heparosan polymer produced by the heparosan synthase, wherein the isolated heparosan polymer is biocompatible with a mammalian patient and biologically inert within extracellular compartments of a mammalian patient, and wherein the isolated heparosan polymer is represented by the structure (-GlcUA-beta1,4-GlcNAc-alpha-1,4-) n, wherein n is a positive integer greater than or equal to 2,000.