- Proteins are made of long chains of amino acids.
- Proteins are composed of carbon, hydrogen, oxygen and nitrogen arranged as the strands of amino acids.
- The digestive system breaks down protein containing food into individual amino acids.
- The body then resembles the amino acids in different orders to make new proteins, which can be used for growth, repairing tissues, hormones and as enzymes.
- As proteins are made of amino acids joined together by peptide bonds amino acids can be called the basic molecules of life.
A strand of amino acids that makes up a protein may contain 20 different kinds of amino acids. Amino acids are the building blocks of proteins. Each has an amine group at one end and an acid group at the other and a distinctive side chain. The backbone is the same for all amino acids. The side chain differs from one amino acid to the next while the nitrogen is in the amine group.
All proteins are made of amino acids and different protein foods contain different proportions of the various amino acids. There are, in all 23 amino acids, in food proteins. As one food cannot supply all the required amino acids, a fair combination of various foods need to be chosen to supply us all the amino acids.
Proteins after digestion are finally broken down into amino acids. Thereafter, they are synthesized for body use. Excess of dietary protein is converted into fat. The food rich in proteins are beef, chicken, pork, turkey, fish, beans, milk, cheese, ice cream, peanut butter, eggs, nuts, cottage cheese and yogurt.
Human body use proteins to build new cells, maintain tissues, and synthesize new proteins that make it possible for performing basic bodily functions. Proteins are present in the outer and linear membranes of every living cell. Proteins play an important part in the creation of every new cell and every new individual. The hair, nails and the outer layers of skin are made of keratin, a scleroprotein, or a protein resistant to digestive enzymes.
To make all the proteins that body needs, it requires 22 different amino acids. Out of these ten are considered essential, which means these cannot synthesize them in body and must obtain them from food. Several more are nonessential. If we don't get them in food, we can manufacture them from fats, carbohydrates and other amino acids.
Glutamine, ornithine and taurine are somewhat in between essential and nonessential for human beings. They are essential under certain conditions such as with injury or disease. The essential and nonessential amino acids are listed below.
| Essential amino acids
Nonessential amino acids
|| Aspartic acid
|| Glutamic acid
|| Hydroxyglutamic acid
The 20 amino acids commonly found in proteins are joined together by peptide bonds. The complexity of protein structure is best analyzed by considering the molecule in terms of four organizational levels, namely primary, secondary, tertiary and quaternary.
Primary structure is the order in which the amino acid are covalently linked together. The primary structure is the one-dimensional first step in specifying the three-dimensional structure of a protein. Understanding the primary structure of proteins is important because many genetic diseases result in proteins with abnormal amino acid sequences, which cause improper folding and loss or impairment of normal function.
Secondary structure is the arrangement in space of the atoms in the peptide backbone. The α-helix and β-pleated sheet arrangements are two different types of secondary structure. In many proteins the folding of parts of the chain can occur independently of the folding of other parts. Such independently folded portions of proteins are referred to as domains or super-secondary structure.
Tertiary structure includes the three-dimensional arrangement of all the atoms in the protein, including those in the side chains and in any prosthetic groups. A specific three-dimensional shape of a protein resulting from interactions between R groups of the amino acid residues in the protein.
Many proteins consist of a single polypeptide chain, and are defined as monomeric proteins. However, others may consist of two or three polypeptide chain that may be structurally identical or totally unrelated. The arrangement of these polypeptide subunits is called the quaternary structure of the protein. All the subunits are held together by the non-covalent interaction like hydrogen bond, ionic bond and hydrophobic bond.
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A protein is a biological macro molecule composed of one or more chain of amino acids linked by peptide bonds. In general, we speak of protein when the string contains more than 50 amino acids. For smaller sizes, we speak of peptide and polypeptide, but more often they are simply "small protein".
The Dutch chemist Gerhard Mulder (1802-1880) discovered proteins. The word protein comes from the Greek "protos" which means first, essential. This probably refers to the fact that proteins are essential to life and they often constitute the majority share (60%) of the dry weight of cells. Another theory that would make reference protein as the adjective protean, with the Greek God Proteus who could change shape at will. The proteins indeed adopt many forms and provide multiple functions. But, this was not discovered until much later, during the twentieth century.
Solubility in Water
- The relationship of proteins with water is complex. The secondary structure of proteins depends largely on the interaction of peptide bonds with water through hydrogen bonds.
- Hydrogen bonds are also formed between protein (alpha and beta structures) and water. The protein-rich static ball are more soluble than the helical structures.
- At the tertiary structure, water causes the orientation of the chains and hydrophilic radicals to the outside of the molecule, while the hydrophobic chains and radicals tend to react with each other within the molecule (cf. hydrophobic effect).
- The solubility of proteins in an aqueous solution containing salts depends on two opposing effects on the one hand related to electrostatic interactions ("salting in") and other hydrophobic interactions (salting out).
A protein is denatured when its specific three-dimensional conformation is changed by breaking some bonds without breaking its primary structure. It may be, for example, the disruption of helix areas. The denaturation may be reversible or irreversible. It causes a total or partial loss of biological activity. This is an important property of protein.
There are a number of Denaturing agents as follows.
- Physical agents: Heat, radiation, pH
Chemical agents: Urea solution which forms new hydrogen bonds in the protein, organic solvents, detergents.