 Hello, welcome back to metals in biology. Today, we will discuss electron transfer in living systems and the book to follow is Principles of Bioinorganic Chemistry by Leapard and Burke. So, lot of schemes figures are taken from this book and also the lecture notes of Professor Leapard ok. So, as you know electron transfer is required to be happening in number of cases in our biological system. All over biological system one electron, two electrons, three electrons or even four electron processes are required. At a time only one electron transfer is usually preferred or without electron transfer certain catalytic cycle cannot be completed. So, therefore, electron the simplest reagent I would say plays a key role in many biological processes. Electron transfer as well as proton transfer usually they are coupled together and that is how they control the redox potential. Often we see that there are metals involved into these electron transfer processes. One of the major criteria that we will see today for the metal center containing electron transfer site is simply before and after electron transfer there should not be too much reorganizational energy. So, the energy of the system before electron transfer and after electron transfer will remain almost constant. So, therefore, metal binding sites are designed in such a way so that they can minimize structural changes upon electron transfer. As I mentioned one electron transfer processes often occurs. So, if let us say you need a multiple electron for a given system one electron at a time will be transferred not two electrons, three electrons or four electrons at a time will be transferred ok. So, it is going to be a step wise electron transfer processes for the multi electron processes. Well electron transfer can happen through bond or through space. In biological system often it is up to 11 to 13 angstrom distance can be covered for the for the electron transfer processes ok. Of course, larger electron transfer or longer electron transfer distance can be also possible we might will not be discussing those too much, but it is also possible to transfer electron over long distances much more than 11 to 13 angstrom. A number of parameters is controlling the electron transfer rate that includes the distance between the origin and the delivery site, the driving force reorganization energy and the path by which the electron transfer happening. So, once again the distance between the donor and acceptor, the driving force and the reorganization energy before and after electron transfer of course, the pathway by which it is going or it is happening that is also will also vary. Today mainly we try to keep it very brief on the electron transfer processes on the iron sulfur clusters, copper centers as well as the heme centers. So, we will see a number of iron sulfur clusters are present for the electron transfer site or as a electron transfer site. Similarly, copper centers can also participate in electron transfer of course, these are all going to be the redox active metal center and that is why they are participating in the electron transfer processes. We can also have heme iron center participating just as electron transfer processes. One of the common thing for these electron transfer centers is they are not directly participating into the main reaction or the main process. If it is a synthetic transformation, they are not actually involved into that except they just provide the electron. If it is just the electron transfer, then only these species come into the picture. Otherwise, they are not really involved too much of the synthetic chemistry or the chemical transformation that happens in the biological system. These are only electron transfer site. Let us look at some of the iron sulfur clusters. Well, these are the most popular one, these are include these includes mononuclear iron center, dinuclear iron center, three iron center and four iron center. So, these clusters can be named in different ways. This is rubridoxin, this is fredoxin which has two iron cluster between the it is bridged by sulphide. As you can see over here for rubridoxin one iron center, these are thiolate or cysteine bound iron center. And for these cases iron is getting oxidized to iron III and it is settling between iron II and III for the electron transfer processes. For these cases, these are two different iron centers and bridged by two sulphide. Of course, the terminal ligands are also sulfur containing cysteine residue of the protein backbone. If you look at this one, this is one of the age of the Cuban is missing. You have 1, 2, 3 iron center bridged by the four sulphide unit 1, 2, 3 and 4. Each of the iron center is also supported by the cysteine thiolate or S minus center. So, overall these are quite fascinating center as well. So, we have finally, 4 iron center bridged by the sulphide and also supported by the cysteine ok. So, these 4 iron cluster center also known as fredoxin, 3 iron cluster center also known as achonitis and these are different overall charges that can they can have during and after the electron transfer processes. Just one thing you must have noticed that these are very solid and bridged structure not too much is going wrong in this system. We will see that in a moment why these are so effective for the electron transfer processes. All before getting into that, let me discuss briefly on the rubredoxin which is a mono iron center. You have one iron and one one iron sulphur distance these are S coming from this cysteine moiety iron sulphur distance any of these we are looking at. The bond distance is usually 2.25 to 2.30 angstrom and we are looking for the oxidized and the reduced form these are the two form by which the electron transfers are happening. After electron transfer from this site it gets oxidized to iron III and once again it accepts one electron and re result in iron II plus formation. Reduction potential is in the range of minus 50 to 50 millivolt for these short of rubredoxin system. So, whenever these reduction potential matches and if these centers are available then these centers can actually provide one electron to the delivery center or the acceptor to the acceptor. As we were discussing there are two iron II sulphur center which are known as ferredoxin. Now these the reduced form of these ferredoxin will contain iron II plus and iron II plus this is fully reduced form and the fully oxidized form would be iron III plus and iron III plus. Obviously, there is a state in between these reduced and oxidized form where one of the iron is in plus II state and another is in plus III state. So, this is for ferredoxin reduction potential for these processes vary from minus 490 to minus 280 millivolt. In this reduction potential range these iron sulphur clusters are operative. We have seen another one this cube missing corner 3 iron IV sulphur ferredoxin you have 3 iron III plus centers 3 plus centers along with the S4, S4 unit as you can imagine that these center can be reduced further this is completely oxidized form and this can be reduced to iron III II and iron II S4. So, one of the iron is getting reduced to iron II plus reduction potential for this for this system varies from minus 700 to minus 100 millivolt. We have seen 4 iron IV sulphur also and that is over here that is also known as ferredoxin and these are also high potential iron proteins and these settles between the 2 sorry 3 iron II plus state and 1 iron III plus state it can be oxidized to 2 of them 2 of them being in iron II plus and 2 of them in iron III plus. Further oxidation of one of these iron II plus leads to only 1 iron II plus form being formed as well as iron III plus 3 of them is remaining ok. So, these are the iron sulphur clusters which are participating in the electron transfer processes and their reduction potential varies from minus 60 minus 650 to minus 280 millivolt region and this is the range where these 4 iron IV sulphur cluster occurs. But most importantly I think we must realize that why these sort of cluster any of those clusters are very effective for the electron transfer process and that is precisely due to the fact that before and after electron transfer there is minimum reorganization energy involved during these process. So, before and after electron transfer things remain as much same as possible. For instant if you see this is Fe 4 S 4 S R 4 minus and in these cases as you can see this is overall 1 minus charge if you add one more electron to this system. So, overall you get 2 minus charge if you add one more electron to this system overall you get 3 minus charge. So, 1 minus 2 minus 3 minus effectively these centers remain same. So, one more electron addition to this leads to this compound one more electron addition to this compound leads to this compound, but irrespective of what electron transfer is happening you see these iron sulphur bond just to pick up one of them these are having crystal structure all 3 of them and this is this distance is iron sulphur distance is being noted as 2.233 angstrom. Whereas, after one electron reduction this distance become 2.267 angstrom which is essentially meaning that this iron sulphur distance really did not change at all or nothing you cannot really say that there is any change and from there on if you add within one more electron these 2.267 angstrom distance between iron and sulphur changes to only 2.354 angstrom. So, virtually these iron sulphur distance remain constant throughout the process and therefore, despite this electron transfer processing processes happening in here there is very little reorganization energy that is required from transforming one species into other. So, this is once again one of the key phenomenon for the electron transfer processes and electron transfer centers which are nothing, but metalloenzymes, but these metalloenzymes remain so solid and so well organized that electron transfer processes does not bother them too much. So, it remains a completely reversible process and they are able to do these chemistry effortlessly and that is what counts a lot ok. In often in nature the processes are so efficient and so simple that that synthetic chemist cannot even really think show so lightly about these processes ok. Well, we have other centers so far you have seen the iron sulphur clusters being the electron transfer site, but then it is not limited electron transfer is not limited to the iron sulphur cluster there are many other centers. For example, there is this copper A center just build up of copper no iron sulphur over here just copper and sulphur is as you can see each of the copper is having a 16 sulphur linkage and they are the bridging one these are the bridging one between the two copper center and one of the terminal is once again methionine which is a copper sulphur bond and the copper or glycine intermediate also over there as you can see each of them are also attached with histidine moiety. Overall it is a dimeric copper center which is which is bridged by this cysteine moiety to cysteine moiety and before once again before and after electron transfer this remains pretty much the same. So, the so the core of the structure did not change at all during and before and after the electron transfer processes ok. Not only these di copper containing copper A centers are responsible for the electron transfer processes there are other centers where also we can see that the major activity or the only activity they carry out is the is that of an electron transfer. Another this series of copper species or another type of copper species is called blue copper that you can see over here over here in the blue copper which is also known as type 1 copper center and the these are having 2 histidine and 1 methionine and 1 cysteine ligation at the copper center. So, this is tetra coordinated copper center and that is over here as you can see it is going to be tetrahedral in nature. So, this is blue copper center that is particularly because these complexes copper 2 complexes copper sulphur charge transfer absorption are quite strong and these charge transfer copper sulphur charge transfer is giving rise to the blue color of these species ok. This is a active site of ascorbate oxidase. Here not only one copper center you have other total 3 copper centers are involved 2 of them are the bridging one which are bridged by the hydroxo these 2 copper center each of them are having 3 histidine in them and they are exchanging or communicating the spin through the hydroxy. So, the EPR spectrum of these actually become EPR silent due to the super exchange right. Now, anti ferromagnetic coupling right these are these are coupled through coupled these 2 copper center are coupled through anti ferromagnetic exchange and therefore, the EPR become silent and in addition to the type 1. So, these are called type 3 copper center which are bridged with each other or bridge and communicated with communicating with each other there is yet another type of copper center which is known as the type 2 copper here you have 2 histidine and 1 OH coordination from the protein backbone. Like overall then it is a 3 different type of types of copper overall total 4 copper these 2 are same this is different this is different. So, ascorbate oxidase is quite an interesting enzyme where you see 3 different types of copper center and overall 4 copper centers are there it is also known as multi copper oxidase. So, these are capable of converting oxygen into water that is fascinating we will see in some classes later that oxygen to water is one of the most fascinating reaction known in the domain of biological and bio inorganic chemistry and these chemistry these oxygen to water transformation chemistry is quite complicated with these copper centers they are remain controversial in this area they are no no generalized mechanism and that tendency can be supported by this ascorbate oxidase so far. So, much more study is required to get the details about the process wherein oxygen is converted into water by utilizing this copper center. Obviously, there are other metal low enzymes which are also capable of converting oxygen into water. So, one such space is that is known so far or one such center that is known so far is methane monoxygenase we will discuss that later ok. Moving on so, as you have seen that these copper centers are distinct. So, this is EPR spectra for copper hexa aqua complex and there is this type 1 copper that is for plastocyanin this is type 2 copper center which are for copper zinc superoxide do you mean dismut as mainly observed the type 1 copper so far you have seen. So, these blue copper centers which are tetra coordinated as you have seen over there these are having very distinct copper EPR as you can see over here these 3 types of copper can be very easily distinguishable by the EPR spectroscopy as I was mentioning this is for the blue copper blue copper center that is the type 1 copper center over here and type 3 is EPR silent and type 2 is the one we have shown over here. Now, this blue copper species also are very active in the UV visible spectra where a strong absorption speak is observed and which gives rise to the blue color of these copper center and by these characteristics blue color one can easily pretty easily isolate these compound by simple column chromatographic technique ok. Moving on it is not only limited to the iron sulfur cluster the electron transfer processes I mean electron transfer processes as you have seen are predominantly controlled by the iron sulfur cluster, but there is also the copper center as you also have seen the copper A site and the blue copper site and again one of the fascinating center for electron transfer processes nothing else just the electron transfer processes in cytochrome C. Just to remind you this cytochrome C is completely different compared to cytochrome C oxidase which we will see later this cytochrome C oxidase is the enzyme which participates in converting oxygen to water also, but then that is the discussion for some other day. Cytochrome C this cytochrome C has only one in heme iron center and the axial ligands are methionine and the histidine these two ligands are axially attached with this heme iron center as you can see over here. So, the porphyrin heme iron in the middle from one axial site it is the methionine and from the other axial site it is the histidine that is bound with it. This is coordinatively saturated and this is why perhaps why it cannot react with oxygen it does not react with anything it is just acting as a stationary center for the electron transfer processes. If the iron center gets oxidized and reduced that is it iron 2 plus and iron 3 plus for the electron transfer processes. So, in these centers all these centers we have discussed so far iron sulfur cluster copper di copper center and the di copper copper center known as the copper A as well as the blue copper centers all and the cytochrome C all of them are capable of transferring one electron at a time and that is what they do contribute towards the active chemistry or active site chemistry of a given metalloenzyme that these are linked with of course, these electron transfer sites are linked with a bigger transformation or bigger processes whenever or wherever that is happening. If you are looking at carefully these centers are going to be a low spin center ok. So, you have a heme iron center and two ligands are there. So, it is a hexacoordinated heme center if you once again notice that these are also designed in such a way so that so that the energy upon upon electron transfer remains called almost reorganization energy remain minimum or these minimum reorganization energy upon electron transfer essentially drives them for the desired electron transfer process ok. So, there is there is the drawing for the protopark for N9 for your heme iron centers as you can see these are also having two axial centers as we have seen in the last slide where we see that a methionine and histidine is linked. So, these methionine and histidine linked iron center will be nothing, but they are low spin. So, this is a very good ligand for firing itself is a very good ligand, but in addition there is methionine and histidine which are strong fin ligand all of them put together it would be a low spin iron complex low spin iron 2 plus complex it is a iron 2 plus is a D6 electronics configuration. So, T2G6 2 2 electron each if you are transferring an electron from these systems. So, iron 2 plus is getting oxidized to iron 3 plus that would be low spin ferric overall then it is going to be T2G5 it is a low spin and therefore, you see that that this electron remains in the T2G level there is no transfer or transfer of electron from the EG level and therefore, the change in the size or the overall reorganization energy is rather minimum for these short of centers. Once again these can only act as one electron transfer processes at a time or these can participate in one electron transfer processes at a time and the main key important things to remember that they have the they have been evolved in such a way so that they minimize the reorganization energy upon electron transfer ok. So, to sum up for this class today I hope you able to see that there are many different electron transfer side depending on the need and the reduction potential that is required for a given transformation. We have seen different various iron sulfur cluster let us say 4 at least 4 different type of iron sulfur cluster. You have seen which is participating in the electron transfer processes there are 1 iron sulfur center, 1 2 iron sulfur center and 3 iron sulfur center as well as the 4 iron sulfur centers. These structure as you have seen in the case of the Cuban structure for the 4 iron 4 sulfur cluster. The crystal structure clearly shows that before and after electron transfer there is essentially nothing changing for this for this cluster and therefore, these rock solid character of this side before and after electron transfer make the most suitable for these processes. There is no reorganization almost no reorganization energy reorganizational energy required for these processes and making them feasible making the electron transfer feasible without much hassle. Not only the iron sulfur cluster you have just seen the porphyrin iron center ligated with 2 ligand such as histidine and the methionine is also capable of transferring electron. This center is called cytochrome C in addition to that there is just copper or copper only system such as copper A and the blue copper center which are having characteristic UV visible and EPR spectra nonetheless these are capable of transferring 1 electron at a time and these are these are pretty good electron transfer side and they are very much valued in the active site chemistry that we will see in the subsequent classes. Keep studying and once again the main book to follow would be Lipper and Bug you can read from any other book of your choice Professor Keim's book is also great. Please do keep studying from different chapters of these books we will see you soon. Thank you very much.