 Dear students, now we are going to review the protein structures chapter. Generally, the protein structures can be classified into four overall types. The first is the primary structure, the second is the secondary structure, then the tertiary structure, and lastly the quaternary structure. The primary structure of proteins is simply the amino acid sequence of each protein. So, how these amino acids they come together defines the protein's primary structure. Once the amino acid chain has formed, then it folds on to itself to create some secondary structures. The secondary structures include alpha helices, pita sheets, loops, coils, and several others. Now, these secondary structures can also come together to create bigger combinations and bigger structures. So, these are the tertiary structures. Typically, a tertiary structure defines the complete structure of a protein. Now, if two proteins they come together and create a complex, then a quaternary structure or the four prime structure is created. There were two generally applicable techniques for measuring the protein structures. So, the first one is the x-ray crystallography and the second one is the NMR spectroscopy. We looked into the details of x-ray crystallography wherein the protein in the sample is crystallized and then x-rays are shot onto the sample and the diffraction that happens from the crystal structure, they are deciphered and the protein structure is calculated. Here, an important point was delivered that the number of protein sequences that we know are far greater in number as compared to the protein structures. So, why is there such a large gap between the known protein structures and the known protein sequences? The simple answer to that was that the structural determination methods are very complex and the proteins that we are trying to structure are easily degraded. So, sometimes a protein may not be crystallized or it may not be soluble. So, how can you determine its protein structure? Of course, there are some experimental strategies to overcome such hurdles, but generally these are the reasons why we only know a very small number of protein structures. So, as I was mentioning, so there were four general types of protein secondary structures, the alpha helices, the beta sheets, the loops and the coils. So, towards predicting the structure of these proteins, the fundamental concept that is employed is the propensity of amino acids to occur within each secondary structure. Now, you just looked at alpha helices from several proteins and then you saw which amino acids were present in the alpha helix. Next, you looked at a beta sheet and then you looked at which amino acids were present in that beta sheet and you repeated this entire process for the entire protein database. So, you arrived at something called the propensities for each amino acid to occur within a specific secondary structure. So, this is a statistical approach towards determining the structure of proteins. Of course, there can be other approaches such as energy-based approaches, which we also studied. Now, towards the algorithm, so we looked at the Chauff-Hassmann algorithm, which is a propensity-based method towards predicting the secondary structures within the proteins. So, provided an amino acid sequence and the propensity for each amino acid to occur in the secondary structures, we could fold that sequence into protein structures. Lastly, we also looked at the protein database for the protein structures. So, it is the most important resource if you are trying to work in protein structures. Of course, there were online tools as well that we looked at such as Modler, which can be used to study the protein structures.