Synthetic iron(II) porphyrin (FeP) is equivalently incorporated into recombinant Thermotoga maritima xylanase B (TMX; family F/10 of glycoside hydrolase), producing a heat-resistant artificial hemoprotein (TMX-FeP) that can bind and release oxygen (O-2) in aqueous medium (pH 7.3, 25 ° C) in the same manner as hemoglobin and myoglobin. The oxygenated species was sufficiently stable; the half-lifetime against the ferric state (τ(1/2)) was 5 h. This O-2-carrying hemoprotein showed a high degree of thermal stability over a wide range of temperatures up to 90 ° C (τ(1/2) = 5 min at 90 ° C and 9 min at 75 ° C). Dictyoglomus thermophilum xylanase B (DTX; family G/11) also incorporates FeP, and DTX-FeP showed identical O-2-binding parameters and thermostability. TMX-FeP is capable of catalyzing the β-1,4-D-xylan hydrolysis reaction. Its larger K-m value compared to that of TMX itself suggested competitive FeP binding to the active site of the host enzyme.