 In this module, we'll talk about protein structure. We'll talk about the primary structure of proteins, secondary structure, tertiary structure, and quaternary structure of protein. The primary structure of protein is basically the sequence in which amino acids are linked together. We have already mentioned that amino acids are basic unit of proteins. They are the monomers. These monomers are linked together with one another through a peptide bond. Peptide bond forms when a water molecule is removed from the two adjacent amino acids, a condensation reaction. The carboxyl group of one amino acid forms a bond. The carbon present in the carboxyl group forms a bond with the nitrogen atom of an adjacent amino acid. This bond, when it forms, a water molecule is lost. So carbon-nitrogen bond is basically the bond that is holding two monomers of a protein together. This bond is called peptide bond. The primary structure, as I mentioned, is basically the result of these peptide bonds, the sequence of different amino acids in a protein molecule. The diversity of different combinations we can form by linking two amino acids is very great. We can take a simple example. If we want to link two amino acids together, we can have 400 different combinations in which we can link two amino acids with each other. As we have mentioned, there are 20 amino acids. So the formula for calculating diversity is 20 to the raised power N. N is the number of amino acids present in a protein. So if you are talking about just two amino acids, it will be 20 to the raised power 2, which is 400. If we are talking about a medium-sized protein, say 100 amino acids long, with this formula, it would be 20 to the raised power 100. This is a very large number. In fact, this number is so large that we don't have enough electrons in this universe. The number of electrons in this universe is less than this number, 20 to the raised power 100. The one thing I also want you to pay attention to is the red oxygen atoms and the blue hydrogen atoms. The hydrogen atom attached to this blue nitrogen atom can form a hydrogen bond with an oxygen atom, which is denoted by a red ball in this diagram. This type of bonding is important in formation of secondary structures. I will highlight these atoms that I'm talking about. Here is the oxygen atom that I mentioned, and here is the nitrogen atom, and this nitrogen atom is attached to the hydrogen atom. This hydrogen atom, the white ball, can form a hydrogen bond with the oxygen atom, and this is important in the secondary structure. There are two types of secondary structures, alpha helices and beta sheets. Alpha helices are present in fibrous proteins. The result from formation of hydrogen bond between the hydrogen atom attached to the nitrogen atom is a peptide bond and also the oxygen atom, which is attached to the carbon atom. The two types, as I mentioned, alpha helices and beta sheets, these are the two types of secondary structures. Tertiary structures basically is the manner in which the whole protein folds upon itself. This tertiary structure can have smaller domains which are composed of secondary structures, alpha helices or beta sheets. This folding of a protein upon itself can be stabilized by all types of different bonds that we have talked about, hydrogen bonds, covalent bonds, the sulfur, we talked about cysteine, sulfur atoms forming the sulfide bond, and also ionic interactions. The quadrary structure basically is more than one protein molecules coming together and forming a larger structure, multi-subunit entity with more than one protein components. Next, we will talk about certain features of the proteins in the next module.