Proteins is a large group of macromolecules. These macromolecules are made up of different twenty amino acids. These amino acids are linked together to form a specific sequence in the form of a chain. There are trillions of possible sequences of these amino acids to form different molecules that are building blocks of the different structures of enzymes and other molecules in living organisms.
These complex structures of proteins are evolved in response to the selection process of billions of years. Proteins are made up of amino acids joined together to form the primary structure. This one-dimensional structure leads to the formation of the 3-dimensional structure of proteins. This 3-dimensional structure enables the proteins to work as an enzyme or other building blocks of different molecules and form this basis of life.
At first, the globular structure of proteins appears as a pile of molecules but it is a well-organized primary structure of amino acids. These amino acids fold and form different peculiar shapes such as loop, helices, etc. These folds form the secondary structure that is formed along the axis of the chain.
These chains fold together and form a compact 3-dimensional structure known as tertiary structure. The tertiary structure has different attractive forces and it is the most stable and biologically active form of proteins. Therefore, this tertiary structure is also known as the native structure of proteins.
These 3 structures exist in all proteins. Some proteins consist of more than one chain of amino acids. These chains fold together and form a compact form of protein known as the quaternary structure of proteins. These structural features of the proteins are the basic reason for their stability. Proteins are flexible and rigid in the function of having hydrophilic and hydrophobic areas in membranes.
Myoglobin showing turquoise α-helices.
Proteins can be distinguished from each other based on the number of amino acids in their chains. The molecular weight of proteins can range from thousand to million Daltons, known as polypeptides. Proteins with low molecular weight having less than fifty amino acids are known as peptides. Each protein has one or multiple polypeptide chains.
Proteins can be classified on the basis of functions and shapes (based in amino acid sequence). These proteins have some common similarities too. Immunoglobulin and hemoglobins are classic examples of protein families. Immunoglobulins are produced by the immune system in response to the invading antigen.
Distantly related proteins are classified into superfamilies. The superfamily of globin protein has various heme-containing proteins that act as a carrier to transport oxygen. It also includes cytoglobin and neuroglobin proteins that are present in the brain and some other tissues.
Of all the molecules that are present in living organisms, proteins are more diverse in structure and functions. Some of its properties are given below.
Enzymes are also known as catalytic proteins. These proteins accelerate several biochemical reactions such as biosynthesis, digestion, and energy synthesis. Enzymes have some remarkable properties.For example, catalytic proteins can enhance the rate of reaction by factor 106 to 1012.
They perform their functions very well under normal conditions of temperature and pH because they can stabilize or induce strained reaction intermediates. For example, In photosynthesis, ribulose bisphosphate carboxylase is very important.
Proteins have very specialized structures that enable them to perform their specific functions such as collagen. Collagen is the major part of connective tissues. Similarly, fibroin has very important mechanical strength. They are found in skin and blood vessels that must be elastic to perform its normal functions.
Proteins form the different important organelles of the cell such as the cytoskeleton, actin, and tubulin that enables the body to perform different movements.
These cytoskeletal proteins perform their role in endocytosis, exocytosis, cell division and ameboid movement of WBCs.
Some of the proteins are protective in nature. For example, in vertebrates, a protein called keratin present in the skin. It protects the organism from different chemical and mechanical injuries.There are some blood-clotting proteins such as thrombin and fibrinogen.
When blood vessels are damaged due to any injury, it prevents blood loss and helps in the healing of the punctured vessels. While lymphocytes produce antibodies (immunoglobulins) protect the organism from any foreign invading bacteria. It binds to the invading bacteria and destroys them.
Proteins also act as a growth factor and bind the different hormones and alter the cellular function. For example, glucagon and insulin regulate the level of glucose in the blood. While growth hormones induce cell division, differentiation and cell growth such as epidermal growth factor and platelet-derived growth factor, etc.
Several proteins act as carriers of various molecules such as ions between cells and membranes. For example, glucose transporter and Na-K ATPase.The major carrier protein is hemoglobin that carries and transports oxygen from the lungs to all body tissues through blood vessels.
Some other carrier proteins are lipoproteins HDL and LDL that carry water-insoluble lipids from the liver. Ceruloplasmin and transferrin are serum proteins that transport copper and iron, respectively.
Proteins enable the living organism to bear a variety of abiotic stresses. For example, cytochrome P450 is a diverse group of enzymes present in plants and animals. It converts the very toxic substances that are very harmful to animals into less toxic substances and their derivatives.
Similarly, a protein known as metallothionein binds the toxic metals including silver, cadmium, and mercury.Animals and plants living in the high-temperature environment have specific proteins known as heat shock proteins help the animals to bear high temperature. DNA repair enzymes protect the cells from radiation.
Recent studies have shown that proteins have multiple and unrelated proteins. It is a diverse class of molecules.
Proteins are also used to store certain essential nutrients. For example, in bird’s eggs store ovalbumin and mammalian milk casein stores organic nitrogen during its development. In plants, a protein named zein performs the same functions as germinating seeds.
- Wolfenden and M. J. Snider. The depth of chemical time and the power of enzymes as catalysts. Acc. Chem. Res., 34(12):938–945, 2001.
- Lolis and G. A. Petsko. Transition-state analogues in protein crystallography: probes of the structural source of enzyme catalysis. Annu. Rev. Biochem., 59:597–630, 1990.
- Gao, S. Ma, D. T. Major, K. Nam, J. Pu, and D. G. Truhlar. Mechanisms and free energies of enzymatic reactions. Chem. Rev., 106(8):3188–3209, 2006.
- Zhang and K. N. Houk. Why enzymes are proficient catalysts: beyond the Pauling paradigm. Acc. Chem. Res., 38(5):379–85, 2005.