To understand the structure-function relationships of a truncated Bacillus sp. strain TS-23 alpha-amylase, each of His-137, His-191, His-239, His-269, His-305, His-323, His-361, His-436, and His-475 was replaced with leucine. The molecular masses of the purified wild-type and mutant enzymes were approximately 54 kDa. The specific activity of His323Leu and His436Leu was decreased by more than 52%, while His239Leu, His305Leu, and His475Leu showed activity similar to that of the wild-type enzyme. As compared with the wild-type enzyme, His323Leu and His436Leu exhibited a 62% decrease in the value of k(cat)/ K(m). Alterations in His-191, His-239, His-305, and His-475 did not cause a significant change in the K(m) or k(cat) values. At 70 degrees C, a decreased half-life was observed in His436Leu. These results indicate that His-137, His-269, and His-361 of Bacillus sp. strain TS-23 alpha-amylase are important for proper catalytic activity and that His-436 may contribute to the thermostability of the enzyme.