The molecular weight is enormous [6] and the structure is extremely entangled and twisted. The problem is that these are macromolecules with a three-dimensional structure that can change the properties of the protein itself if even one of them changes, so it is impossible to express it only with element symbols and lines like other substances.
Despite the completion of the human genome project, the expected results are not coming out because the types of proteins that can be made by alternative splicing are not directly proportional to the information of the genome [7]. In addition, because of the complex structure (twist) of protein macromolecules, it is almost impossible to infer the function of a protein using only genetic information. It is impossible to know what three-dimensional shape the protein molecule, which is the actual product, will have by simply knowing the sequence of amino acids,[8] because this shape is an important factor in determining the function of protein molecules (enzymes).[9] For this reason, the weight of related studies is shifting from genomics, which was only focused on gene sequences, to proteomics, which studies the shape of protein molecules.
Basically, it is known that the more disulfide bonds (-S-S-; Disulfide Bond) and the more cyclical it is, the more stable it is in the body.[10]
4.1. Order of protein structures[edit]
The structure of a protein refers to the order in which peptide-linked amino acids are arranged. The primary structure is simply a sequence of amino acids, so it is called a polypeptide. Unlike DNA, proteins have other unique structures besides sequences.
Secondary structure refers to what structure amino acids form locally. Alpha spirals and beta screens are typical. Since it refers to the appearance formed by the local hydrogen bond between the amine group and the carboxyl group [11] between amino acids, rather than the entire protein, several types of secondary structures can appear in one protein.
Tertiary structure refers to the three-dimensional structure of the entire sequence, rather than partial, like secondary structures. It is mainly formed by hydrogen bonds between R groups of different amino acids, interactions between methane groups, van der Waals forces, disulfide bonds [12], and ionic bonds. From here, it can be called a single protein rather than a simple amino acid or polypeptide 'unit' and represents the unique function of the protein.
There is also a quaternary structure. This refers to how several proteins come together to form a complex. Examples include RNA polymerase, which is used to transcribe RNA from DNA. It is a complex made up of an enormous number of proteins.[13]
4.2. Structural prediction[edit]
It is a high-molecular organic substance that uses amino acids as monomers, and is formed by combining single or multiple peptides formed by polymerization of amino acids. There are a total of 20 amino acids that make up proteins in living organisms. One of the characteristics of proteins is that they have various structures that are difficult to predict, depending on the type and order of these amino acids. See the protein folding article for more details.
Usually, hundreds to thousands of amino acids are included per peptide unit. A protein composed of many amino acids can have a structure that is difficult for humans to imagine.[14] Thousands of scientists struggle to find useful proteins, but they are so numerous that they will continue to find them.
As of April 2020, the structures of 162,816 types of proteins have been identified. # Of course, this is not all. [15]
5. Classification according to composition[edit]
Protein is not just a chain of amino acids that are twisted and folded. There is also a complex protein in which other molecules [16] are attached to the protein, and the hemoglobin immediately above is also a type of complex protein, a metalloprotein. Metalloproteins because the iron in the heme group is a metal. Similarly, because of the red heme group because of iron oxide, it also belongs to pigment proteins. There are about 7 types of complex proteins.
Nucleoprotein: A protein associated with a nucleic acid. A typical example is a histone protein.
Glycoprotein: A protein with sugars attached to it. The sugar content is less than 4%.
Viscous protein: Like glycoproteins, sugars are added, but when the content is more than 4%, it is called viscous protein. The famous glucosamine belongs to this category.
Lipoprotein: A protein bound to a lipid. Representatively, there are proteins constituting the phospholipid bilayer of animal cells and the myelin sheath of neurons.
Phosphoprotein: A protein with around 1% nucleic acid other than phosphoric acid or phospholipid.
Metalloprotein: At this point, it's obvious, but a protein with a metal. Examples include amylase and hemoglobin.
Chromoprotein: A protein with pigments. Similarly, hemoglobin belongs to this category as the heme group is red because of iron.
Considering that different prosthetics can run on each of the hundreds of thousands of protein types above, the types of proteins are virtually infinite.