Basic unit of protein and its function

The word protein is derived from Greek word, “proteios” which means primary. As the name shows, the proteins are of paramount importance for biological systems. Proteins are biochemical molecules consisting of polypeptides joined by peptide bonds between the amino and carboxyl groups of amino acid residues.

Proteins are made up of hundreds or thousands of smaller units known as amino acids. Most organisms use 20 naturally-occurring amino acids to build proteins. The linear sequence of the amino acids in a protein is dictated by the sequence of the nucleotides in an organisms’ genetic code. Amino acids can combine to form long linear chains known as polypeptides. Each type of polypeptide chain has a unique amino acid sequence.

The sequence of amino acids determines each protein’s unique 3-dimensional structure and its specific function such as catalysis of biochemical reactions, mechanical support and immune protection, movement, transport of ligand, transmits nerve impulses, and control growth and differentiation.

The proteins function to regulate specific steps in metabolism – one step, one protein. Hence, many proteins are needed.

The polypeptide must fold into a specific three-dimensional structure before it can perform its biological functions. The function of all proteins depends on their ability to specifically interact with other molecules. Such specificity is possible because polypeptides with different amino acid sequences fold into different tertiary structures.

Proteins are not entirely rigid molecules. They undergo conformational changes upon ligand binding. Each kind of protein evolved to interact with a specific molecule or ligand. For example, transport proteins (such as hemoglobin) bind to specific ligands (in this case oxygen) and transport the ligand to a site where it is needed. Hemoglobin, the transporter of oxygen is a tetrameric protein (alpha 2, beta 2), with each monomer having a heme unit. Binding of oxygen to one heme facilitates oxygen binding by other subunits.

Storage proteins such as myoglobin, another oxygen-binding protein, allow the cell to store higher concentrations of the ligand than otherwise would be possible.

Catalytic proteins— the enzymes—convert the ligands into other molecules. They act as biochemical catalysts. The first step in enzymatic catalysis is the binding of the enzyme to the substrate. This, in turn, depends on the structural conformation of the active site of the enzyme, which is precisely oriented for substrate binding

Many proteins have structural or mechanical functions. Structural proteins interact with specific molecules, often endowing the bound molecules with special biological properties. For instance, one class of proteins, the histones, binds to DNA to form compact nucleoprotein structures called nucleosomes, while a second class of proteins combines with RNA to form the ribonucleoprotein complex known as the ribosome.

Structural proteins collagen is the most abundant protein in mammals and is the main fibrous component of skin, bone, tendon, cartilage and teeth.

Proteins are also important in cell signaling, immune responses, cell adhesion, and the cell cycle.
Basic unit of protein and its function

 

What is biological value of protein?

Since proteins are of such great importance to animals and man, many plants are grown because of nutritional value of their proteins.

However not all proteins have the some biological value. Biological value can be defined as the percentage of the absorbed nitrogen retained in the body.

Protein quality refers to the ability of a dietary protein to supply the amino acid needs of the body. The fact that a specific food is a rich source of protein does not indicate that the food has any particular value in supporting growth or maintenance. Some proteins are rich in certain essential amino acids, and thus have a high biological value, whereas other proteins are devoid of some of these amino acids or contain them in very small amounts.

For example, gelatin is a protein that is sometimes used in cooking. This protein is available in a pure powdered form; however, the use of gelatin as a food and as the sole source of protein cannot supply the body’s amino acid needs.

In those areas of the world where the main protein source is vegetable protein that lacks certain amino acids, protein deficiency diseases often occur, particularly in children.

 A protein with biological value of 70 or more is considered capable of supporting growth, assuming caloric value of the diet is adequate. This means that 70% of the nitrogen absorbed is retained.
What is biological value of protein? 

Structure of protein

In 1960, the British biochemist John Kendrew used a method called ‘X-ray diffraction’ to photograph myoglobin at a 2 A resolution and became the first man to determine the three-dimensional structure of a protein.

The basic structure common to all proteins is the peptide linkage which is formed by condensation the carboxyl group of one of amino acid with the amino group of another.

In this way chains are created, which contains only 3 amino acids, to complex polymers of 1000 or more. At physiological temperatures in aqueous solution, the polypeptide chains of proteins fold into a form that in most cases is globular.

The sequence in which amino acids are arranged in the peptide chain is known as the primary structure of the molecule. In biochemistry, this is always given starting with the N-terminal and ending with the C-terminal amino acid, because this is the order in which amino acids are added during protein synthesis in the cells.

The amino acid chain is the primary and central component of the protein, but not necessarily the only component. Some protein may include other atoms or small molecules which are required for their function and. or stability. The proper sequence of amino acids tends to be a critical factor in protein function.

Protein molecules serve as some of the major structural elements of living system. This function depends on specific association of protein subunits with themselves as well as with other proteins, carbohydrates and so on, enabling even complex systems like actin fibril to assemble spontaneously.
Structure of protein

Amino Acids

Proteins are sequences of amino acids. All amino acids contain at least one amino group (-NH2) in the alpha position and one carboxyl, and all (except Glycine) contain an asymmetric carbon atom. For this reason, they may exist as isomers.

There are 20 different amino acids each consisting of a backbone to which a side group is attached. The amino acid backbone is the same for all amino acids, but the side group varies. It is side group that makes each amino acid unique.

Most naturally occurring amino acids are of the L-configurations, although D-amino acids are not uncommon in some microorganisms.

The presence of a D-amino acid oxidase in mammalian tissue, however, suggests that the D-forms may play some yet unrecognized role in mammalian protein metabolism.

Nine of amino acids are called essential amino acids, because human body cannot make them and must get them though diet. The body can manufacture the remaining 11 amino acids, called nonessential amino acids. 

When amino acid backbones joined end to end, a protein forms. The bonds that from between adjoining amino acids are called peptide bonds. Proteins often contain from 35 to several hundred or more amino acids.

Amino acids are not stored in the body in any appreciable amounts; therefore, proper nutrition requires eating enough protein just about every day to meet the body’s needs for essential amino acid.
Amino Acids