Biochemistry and Comparative Genomics of SxxK Superfamily Acyltransferases Offer a Clue to the Mycobacterial Paradox: Presence of Penicillin-Susceptible Target Proteins versus Lack of Efficiency of Penicillin as Therapeutic Agent

The bacterial acyltransferases of the SxxK superfamily vary enormously in sequence and function, with conservation of particular amino acid groups and all-α and α/β folds. They occur as independent entities (free-standing polypeptides) and as modules linked to other polypeptides (protein fusions). They can be classified into three groups. The group I SxxK d,d-acyltransferases are ubiquitous in the bacterial world. They invariably bear the motifs SxxK, SxN(D), and KT(S)G. Anchored in the plasma membrane with the bulk of the polypeptide chain exposed on the outer face of it, they are implicated in the synthesis of wall peptidoglycans of the most frequently encountered (4→3) type. They are inactivated by penicillin and other β-lactam antibiotics acting as suicide carbonyl donors in the form of penicillin-binding proteins (PBPs). They are components of a morphogenetic apparatus which, as a whole, controls multiple parameters such as shape and size and allows the bacterial cells to enlarge and duplicate their particular pattern. Class A PBP fusions comprise a glycosyltransferase module fused to an SxxK acyltransferase of class A. Class B PBP fusions comprise a linker, i.e., protein recognition, module fused to an SxxK acyltransferase of class B. They ensure the remodeling of the (4→3) peptidoglycans in a cell cycle-dependent manner. The free-standing PBPs hydrolyze d,d peptide bonds. The group II SxxK acyltransferases frequently have a partially modified bar code, but the SxxK motif is invariant. They react with penicillin in various ways and illustrate the great plasticity of the catalytic centers. The secreted free-standing PBPs, the serine β-lactamases, and the penicillin sensors of several penicillin sensory transducers help the d,d-acyltransferases of group I escape penicillin action. The group III SxxK acyltransferases are indistinguishable from the PBP fusion proteins of group I in motifs and membrane topology, but they resist penicillin. They are referred to as Pen(r) protein fusions. Plausible hypotheses are put forward on the roles that the Pen(r) protein fusions, acting as l,d-acyltransferases, may play in the (3→3) peptidoglycan-synthesizing molecular machines. Shifting the wall peptidoglycan from the (4→3) type to the (3→3) type could help Mycobacterium tuberculosis and Mycobacterium leprae survive by making them penicillin resistant.

SxxK超家族细菌酰基转移酶的序列与功能差异悬殊，但特定氨基酸基团以及全α螺旋、α/β折叠结构具有保守性。它们既可以以独立实体（游离多肽）的形式存在，也可作为模块与其他多肽融合形成融合蛋白。该家族可分为三类：Ⅰ型SxxK d,d-酰基转移酶在细菌界广泛分布，均携带SxxK、SxN(D)及KT(S)G保守基序，锚定在细胞膜上且多肽链大部分暴露于细胞膜外表面，参与最常见的(4→3)型细胞壁肽聚糖的合成；青霉素及其他β-内酰胺类抗生素可将其灭活，此类抗生素以青霉素结合蛋白（PBPs）的形式作为自杀性羰基供体发挥作用。Ⅰ型SxxK d,d-酰基转移酶属于形态发生装置的组成部分，该整体可调控细胞形状、大小等多项参数，使细菌细胞得以生长并复制其特定形态模式。其中A型PBP融合蛋白由糖基转移酶模块与A型SxxK酰基转移酶融合而成，B型PBP融合蛋白由连接模块（即蛋白质识别模块）与B型SxxK酰基转移酶融合而成，二者均以细胞周期依赖的方式介导(4→3)型肽聚糖的重塑，游离型PBPs可水解d,d肽键。Ⅱ型SxxK酰基转移酶常带有部分修饰的基序条形码，但SxxK基序始终保守，它们可与青霉素以多种方式结合，体现了催化中心极高的可塑性；分泌型游离PBPs、丝氨酸β-内酰胺酶以及多种青霉素传感转导子的青霉素受体，可帮助Ⅰ型d,d-酰基转移酶逃避青霉素的抑制作用。Ⅲ型SxxK酰基转移酶在基序与膜拓扑结构上与Ⅰ型PBP融合蛋白无差异，但可耐受青霉素，因此被称为Pen(r)融合蛋白。本文提出合理假说：作为l,d-酰基转移酶的Pen(r)融合蛋白，可能参与(3→3)型肽聚糖合成的分子机器；将细胞壁肽聚糖从(4→3)型转变为(3→3)型，可使结核分枝杆菌与麻风分枝杆菌获得青霉素耐药性，从而帮助其存活。