Unpolish Rice And Fingermillets Nutritions

Plants can synthesize amino acids they need from raw materials in their cells, but animals have to supplement from plant sources some amino acids they need, since they can manufacture only a few amino acids themselves.

Each polypeptide usually coils, bends, and folds in a specific fashion within a protein, which characteristically has three levels of structure and sometimes four:

1. A sequence of amino acids fastened together by peptide bonds forms the primary structure of a protein.

2. As hydrogen bonds form between oxygen and hydrogen atoms or nitrogen and hydrogen atoms of different amino acids, the polypeptide chain coils like a spiral staircase, and the secondary structure develops. Some secondary structures include polypeptide chains that double back and form hydrogen bonds between the two lengths in what is referred to as a beta sheet, or pleated sheet.

3. Tertiary structure develops as the polypeptide further coils and folds. The tertiary structure is maintained by bonds between R groups.

4. If a protein happens to have more than one kind of polypeptide, a fourth, or quaternary structure, may form (Fig. 2.13).

The three-dimensional structure of a protein may be somewhat flexible in solution, but chemicals or anything that disturbs the normal pattern of bonds between parts of the protein molecule can denature the protein. Denaturing alters the characteristic coiling and folding and adversely affects the protein's function or properties. Denaturing may be reversible, but if it is brought about by high temperatures or harsh chemicals, it may kill the cell of which the protein is a part. For example, boiling an egg, which is mostly protein, brings about an irreversible denaturing; the solid egg proteins simply can't be restored to their original semiliquid condition.

Storage Proteins Some plant food-storage organs, such as potato tubers and onion bulbs, store small amounts of proteins in addition to large amounts of carbohydrates. Seeds, in particular, however, usually contain proportionately larger amounts of proteins in addition to their complement of carbohydrates and are very important sources of nutrition for humans and animals. One example of an important protein source in human and animal diets is wheat gluten (to which, incidentally, some humans become allergic). The gluten consists of a complex of more than a dozen different proteins.

A seed's proteins get used during germination and its subsequent development into a seedling. Some legume seeds may contain more than 40% protein, but legumes are deficient in certain amino acids (e.g., methionine), and a human diet based on beans needs to be balanced with other storage proteins (e.g., those found in unpolished rice) to furnish a complete complement of essential amino acids. Some seed proteins, such as those of jequirity beans (Abrus precatorius—used in India to induce abortions and as a contraceptive), are highly poisonous.

Enzymes Enzymes are mostly large, complex proteins (a few are ribonucleic acids, which are discussed on p. 26) that function as organic catalysts under specific conditions of pH and temperature. By breaking down bonds and allowing new bonds to form, they facilitate cellular chemical reactions, even at very low concentrations, and are absolutely essential to life. None of the 2,000 or more chemical reactions in cells can take place unless the enzyme specific for each one is present and functional in the cell in which it is produced. They can increase the rate of reaction as much as a billion times, and without them, the chemical reactions in cells would take place much too slowly for living organisms to exist. Enzymes are often used repeatedly and usually do not break down during the reactions they accelerate.

Enzyme names normally end in -ase (e.g., maltase, sucrase, amylase). The material whose breakdown is catalyzed by an enzyme is known as the substrate. Maltose is a very common disaccharide composed of two glucose monomers. The enzyme maltase catalyzes the hydrolysis of maltose (its substrate) to glucose. Enzymes work by lowering the energy of activation, which is the minimal amount of energy needed to cause molecules to react with one another. An enzyme brings about its effect by temporarily bonding with potentially reactive molecules at a surface site. The reactive molecules temporarily fit into the active site, where a short-lived complex is formed. The reaction occurs rapidly, often at rates of more than 500,000 times per second. The complex then breaks down as the products of the reaction are released, with the enzyme remaining unchanged and capable of once more catalyzing the reaction (Fig. 2.14).

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