The paradigm that protein structure determines protein function has been clearly established. What is less clear is whether a specific protein structure is always required to carry out a specific function. Numerous cases are now known where there is no apparent connection between the biological function of a protein and the other members of its structural class, and where functionally related proteins can have quite diverse structures. A set of enzymes with these diverse properties, the ammonia-lyases, will be examined in this review. These are a class of enzymes that catalyze a relatively straightforward deamination reaction. However, the individual enzymes of this class possess a wide variety of different structures, utilize a diverse set of cofactors, and appear to catalyze this related reaction through a range of different mechanisms. This review aims to address a basic question: if there is not a specific protein structure and active site architecture that is both required and sufficient to define a catalyst for a given chemical reaction, then what factor(s) determine the structure and the mechanism that is selected to catalyze a particular reaction?

蛋白质结构决定蛋白质功能的范例已经明确建立。尚不清楚的是，是否始终需要特定的蛋白质结构来执行特定的功能。现在已知许多情况，其中蛋白质的生物学功能与其结构类别的其他成员之间没有明显的联系，而功能相关的蛋白质可以具有非常不同的结构。在这篇综述中，将研究一组具有这些不同特性的酶，即氨分解酶。这些是催化相对简单的脱氨反应的一类酶。然而，这类的单个酶具有各种各样的不同结构，利用了各种各样的辅因子，并且似乎通过一系列不同的机制来催化这种相关的反应。