The expression of β-lactamases is a major mechanism of bacterial resistance to the β-lactam antibiotics. Four molecular classes of β-lactamases have been described (A, B, C and D), however until recently the class D enzymes were thought to exist only in Gram-negative bacteria. In the last few years, class D enzymes have been discovered in several species of Gram-positive microorganisms, such as Bacillus and Clostridia, and an investigation of their kinetic and structural properties has begun in earnest. Interestingly, it was observed that some species of Bacillus produce two distinct class D β-lactamases, one highly active and the other with only basal catalytic activity. Analysis of amino acid sequences of active (BPU-1 from Bacillus pumilus) and inactive (BSU-2 from Bacillus subtilis and BAT-2 from Bacillus atrophaeus) enzymes suggests that presence of three additional amino acid residues in one of the surface loops of inefficient β-lactamases may be responsible for their severely diminished activity. Our structural and docking studies show that the elongated loop of these enzymes severely restricts binding of substrates. Deletion of the three residues from the loops of BSU-2 and BAT-2 β-lactamases relieves the steric hindrance and results in a significant increase in the catalytic activity of the enzymes. These data show that this surface loop plays an important role in modulation of the catalytic activity of Bacillus class D β-lactamases.

β-内酰胺酶的表达是细菌对β-内酰胺类抗生素产生耐药性的主要机制。已经描述了四种分子类别的 β-内酰胺酶（A、B、C 和 D），但是直到最近，人们认为 D 类酶仅存在于革兰氏阴性菌中。在过去的几年中，已经在多种革兰氏阳性微生物中发现了 D 类酶，例如芽孢杆菌属和梭状芽孢杆菌属，并且已经开始认真研究它们的动力学和结构特性。有趣的是，观察到某些芽孢杆菌属产生两种不同的 D 类 β-内酰胺酶，一种具有高活性，另一种仅具有基础催化活性。活性氨基酸序列分析（来自短小芽孢杆菌的BPU-1) 和无活性（来自枯草芽孢杆菌的BSU-2和来自萎缩芽孢杆菌的BAT-2 ）酶表明低效 β-内酰胺酶的表面环之一中存在三个额外的氨基酸残基可能是其活性严重降低的原因。我们的结构和对接研究表明，这些酶的拉长环严重限制了底物的结合。从 BSU-2 和 BAT-2 β-内酰胺酶的环中删除三个残基可减轻位阻并导致酶的催化活性显着增加。这些数据表明，该表面环在调节芽孢杆菌D 类 β-内酰胺酶的催化活性中起重要作用。