Function and regulation of mitochondrial cytochrome c oxidase using mutant forms of the yeast enzyme

线粒体是我们细胞的主要能量提供者。它们有一条蛋白质链——线粒体呼吸链——嵌在它们的内膜中，氧化食物代谢产生的产物，并将氧气还原为水。细胞色素c氧化酶（CcO）是该链的最后一个组分，催化氧还原反应；事实上，正是这种酶消耗了我们呼吸的大部分氧气，它提供了我们所需能量的很大一部分。在与氧的反应过程中，能量被释放出来，而CcO能够通过在线粒体膜上产生电荷差来捕获能量，这相当于给电子产品中的电容器充电。这种电荷梯度随后被另一种称为ATP合酶的酶利用来制造ATP。ATP是一种稳定的化学物质，水解时可以释放能量；它在细胞周围扩散，并为我们的细胞和组织的健康运作所需的无数反应提供大部分能量。我们已经对酶的原子结构以及它与氧安全反应释放能量的方式有了很大的了解。我们也很好地理解了它利用能量来形成电荷梯度的一般方法。然而，为了阐明耦合机制的具体细节，我们不得不严重依赖于对酶的简单细菌形式的研究，特别是通过引入突变来改变这些细菌酶的单个部分的能力。然而，这种细菌酶的研究不能解决我们人类更复杂的CcO如何工作的所有方面。几种不同类型的细菌CcO的结构是已知的，但迄今为止，唯一已经解决的线粒体CcO结构是来自牛心脏的CcO。尽管所有形式的CcO在氧气减少的区域周围都有相似的核心结构，但细菌和哺乳动物的CcO之间的其他结构差异导致人们认为能量储存反应与细菌的CcO不同。一个更显著的区别是，线粒体CcOs有多达10个额外的亚基，这些亚基在细菌CcOs中都没有发现，迄今为止我们对它们的功能知之甚少。也有充分的证据表明，与细菌的CcOs相比，在更复杂的生物体中，这些CcOs可以以不同的形式发生，并在不同组织的细胞环境中受到不同的调节。了解这些方面是很重要的，因为一系列人类遗传疾病都是由CcO功能失调引起的。不幸的是，由于缺乏引入突变的良好系统，这些独特的线粒体CcO问题的研究受到严重阻碍。值得注意的是，在贝克酵母细胞线粒体中存在的CcO在其核心结构和所有附加亚基的结构方面与人类CcO线粒体在结构上极其相似。正因为如此，也因为我们可以对酵母的部分CcO进行改变，就像对细菌的CcO所做的一样，我们现在可以研究这些线粒体（因此也是人类）的CcO是如何起作用的，以及如何被控制的。了解更复杂的线粒体酶的独特方面将为测试和了解人类酶在健康和疾病中的功能和故障提供一个平台。

Mitochondria are the major energy providers of our cells. They have a chain of proteins - the mitochondrial respiratory chain - embedded in their inner membrane that oxidise products derived from the metabolism of food and reduce oxygen to water. Cytochrome c oxidase (CcO) is the final component of this chain that catalyses the oxygen reduction reaction; in fact, it is this enzyme that is responsible for consumption of most of the oxygen that we breathe and it provides a significant fraction of our energy needs. Energy is released during its reaction with oxygen and CcO is able to trap it by creating a charge difference across the mitochondrial membrane in which it is located - rather equivalent to charging of a capacitor in electronics. This charge gradient is then be used by another enzyme, called ATP synthase, to make ATP. ATP is a stable chemical that can release energy when it is hydrolysed; it diffuses around the cell and supplies a large part of energy needed by the myriad of reactions required for healthy functioning of our cells and tissues.We already know a great deal about the atomic structure of the enzyme and the way in which it safely reacts with oxygen to release energy. We also have a very good idea of the general way that it uses the energy to make the charge gradient. However, in order to elucidate the specific details of the coupling mechanism we have had to rely heavily on studies of simpler bacterial forms of the enzyme, particularly aided by the ability to change individual parts of these bacterial enzymes by introducing mutations. However, such bacterial enzyme studies cannot address all aspects of how our own human, more complicated, CcO works. The structures of several different types of bacterial CcOs are known but, to date, the only mitochondrial CcO structure that has been resolved is that from cow hearts. Although all forms of CcO share a similar core structure around the region where oxygen in reduced, other structural differences between bacterial and mammalian CcOs have led to suggestions that the energy storage reactions are different from those seen in bacterial CcOs.. An even more striking difference is that mitochondrial CcOs have up to 10 additional subunits, none of which are found in bacterial CcOs and to date we have little understanding of their functions. There is also good evidence that, in contrast to bacterial CcOs, those in more complex organisms can occur in different forms and are regulated differently in the cellular environments of different tissues. It is important to understand these aspects because a range of human genetic diseases arise from malfunctioning of CcO. Unfortunately, studies of these uniquely mitochondrial CcO questions are severely hampered by the lack of a good system to introduce mutations.Remarkably, the CcO that is present in mitochondria of Baker's yeast cells is structurally extremely similar to human CcO mitochondria in terms of both its core structure and the structures of all of its additional subunits. Because of this, and because we can make alterations in parts of yeast CcO in the same way as has been done successfully with bacterial CcOs, we can now investigate just how these mitochondrial (and therefore human) CcOs function and can be controlled. The understanding of the unique aspects of the more complex mitochondrial enzymes will provide a platform for testing and understanding how the human enzyme functions and malfunctions in health and disease.