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Proteins are the polymers of amino acids in which amino acids are linked together through an amide bond between a α-carboxyl carbon of the first amino acid and the α-amino nitrogen of the next amino acid. This type of amide bond is called as a peptide bond. Depending on the number of amino acids linked, they can be classified as a dipeptide, tripeptide, tetrapeptide or so on. However, polypeptides with molecular weight 10 kDa or higher are called as proteins.
A protein may be composed of a single polypeptide or of more than one polypeptides. Proteins with more than one polypeptide are called as proteins with sub-units, where each polypeptide represents separate sub-units. There are four levels of protein structure architectures. Primary structure (the linear sequence of the polypeptide), secondary structure (α-helix and β-pleated sheets), tertiary structure, and the quaternary structure.
Tertiary structure is the most common structure of all the proteins while the quaternary structure is common only to the proteins which are made of more than one polypeptide chain. The highest level of structure of proteins with single polypeptide chain is the tertiary structure while the highest level of structural architecture of proteins with more than one polypeptide is the quaternary structure.
Well, it is quite difficult to say how many different kinds of proteins present in a single cell because each cell in an organism contains the same DNA (same number of genes) that are responsible for the expression of all types of the protein. That is, all cells have the same genetic information for the protein expression, however, which protein to express and which protein not to express is determined by the current need of the cell. For example, if a certain protein is needed at a time, then the gene for that protein is turned on otherwise remains turned off.
There are certain types of proteins (house-keeping proteins) that are required throughout the time to perform a certain function and, therefore, their expression happens throughout the time. Most of these housekeeping proteins are the enzymes that are needed for the metabolism throughout the time.
Mainly, there are two types of DNA; genomic DNA and the plastid DNA (mitochondrial DNA and in plants chloroplast DNA). They contain different sets of genes which are expressed separately. Plastid genes are expressed to respective proteins that are useful for the plastid while genomic gene expression produces proteins required for the whole cells and organisms too.
Let’s take an example, in the cells of adipose tissues; different kinds of proteins are expressed some of which are enzymes responsible for the metabolism of lipids, carbohydrates, etc. Some proteins are in the form of initiation factors, elongation factors and other regulators involved in the regulation of gene expression. Moreover, some regulatory proteins are also produced which are responsible for the downstream signal transduction from the cell surface to the intracellular organelles and vesicles. Some proteins are in the form of channels located in the membrane to allow different biomolecules to pass selectively through the membrane.
Thus, a single cell contains thousands of types of proteins and each of them has their own roles and functions to perform inside the cell. That keeps the cell alive and active. However, not all the proteins are expressed at a time but are expressed according to the current need of the cell.