PROTEINS

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PROTEINS


•       Proteins are large, complex, organic compounds made up of carbon, hydrogen, oxygen, and nitrogen

•       Basic material of every living cell

•       Only nutrient that can make new cells and rebuild tissues

•       Amino acids are the basic units of it

Each amino acid contains a carboxyl group (COOH) or acid group and an amino group (NH2) or basic group

•       Proteins consist of chains of amino acids that are linked to each other by a peptide linkage (-CO-NH-)

•       Protein (containing 16% nitrogen) content measured by nitrogen content. Hence,1 gm nitrogen = 6.25 g protein

Essential Amino Acids

•       Cannot be synthesized in sufficient amounts by body

•       Must be provided by diet, hence very essential for human beings.

•       Human adult requires 8 whereas growing children require 10 essential amino acids.

•       Examples: Isoleucine, leucine, lysine, methionine, tryptpphan, valine, histidine, arginine, etc.

Non-essential Amino Acids

•       Non-essential amino acids doesn’t mean that these amino acids are not required by body

•       Termed non-essential because they are not dietary essentials

•       If lacking in diet, will be synthesized by body from other amino acids

•       Examples: Alanine, asparagine, cystine, glutamine, glutamic acid, glycine, proline, serine, etc.

Classification of Proteins

q Classification by Structure

•       Simple proteins: Proteins made with only amino acids, e.g. zein in corn, albumin in egg white, and gliadin in wheat

•       Conjugated proteins: Proteins having non-protein molecule attached to protein, e.g. Blood protein haemoglobin, which contains a haeme (iron) group attached to protein

•       Derived proteins: Resulting from partial breakdown of a native protein. Proteoses, peptones, and polypeptides are formed when digestive enzymes begin their action on proteins.

q Classification by Quality

•       Complete proteins: Proteins containing all essential amino acids in right proportion to meet the body’s need for growth a of repair of tissue cells. E.g. Eggs, milk, meat, fish, poultry. Found in animal foods.

•       Partially complete proteins: Proteins in which one or more essential amino acids are present in inadequate amounts. Cannot synthesize tissues w/o help of other proteins.  E.g. Cereals, pulses, nuts and oilseeds. Found in plant foods.

•       Incomplete proteins: Proteins incapable of growth and repair of body cells. Cannot maintain life. One or more essential amino acids may be completely lacking. E.g. Gelatin lacking 3 essential amino acids.

Functions of Protein

•      Structural Function

q  Growth: Primary function is synthesis of body cells. All body tissues and fluids except urine and bile are made up of protein. Proteins are the major constituent of muscles, organs, endocrine glands, and collagen. Collagen is the main structural protein of bones, tendons, ligaments, skin, blood vessels and connective tissues. Proteins are required for the growth and formation of these substances.

q  Maintenance of wear and tear: It is required by all age groups for continuous maintenance of all cells in the body. Proteins required to replace the old and worn out cells.

•      Regulatory Function

Certain amino acids and proteins have highly specialized functions in the regulation of body processes and protection against disease.

q  Haemoglobin, an iron containing protein in the red blood cells, performs an important role by transporting oxygen to the tissue cells

q  Plasma proteins maintain water balance and regulate osmotic pressure in the body

q  Antibodies (neutralizes pathogens) perform a protective function by increasing body’s resistance to disease

q  All enzymes and some hormones, e.g. Insulin are made of protein which regulates blood sugar levels

q  Some amino acids have specific functions e.g. Tryptophan serves as a precursor for niacin, a B-complex vitamin

•      Energy

q  Like Carbohydrates, protein too provides 4kcal/g when broken down in the body

q  If the diet doesn’t supply adequate calories from carbohydrates and fats, the proteins from diet will be oxidized to meet the energy needs of body

q  Protein is used by the body as a source of energy only when no other source of energy is available

q  If the diet is deficient in calories, the body uses up its protein and fat stores

Methods of improving Protein Quality

Animal proteins contain all essential amino-acids in right quantity.

Plant proteins are short of 4 essential amino acids such as: lysine, methionine, threonine and tryptophan.

- Protein quality of a meal can be improved by following methods:

•       By incorporating a small quantity of complete protein food in every meal. Complete protein foods such as milk, curds, paneer, cheese, buttermilk and eggs could be used in small quantities in various dishes rather including it in one meal only, e.g. Cereal and milk, egg or cheese sandwiches

•       Correct mixtures of plant foods could provide all essential amino acids in suitable proportions and amount. Cereal and pulse combinations will complement each other as cereals provide methionine, which is lacking in pulses, and pulses provide lysine, which is lacking in cereals, e.g. Missi roti, idli, puran poli, rajma chawal etc.

Therefore, it means:

Cereal + Pulses + Green leafy vegetable = Flesh Foods

•       Synthetic amino acids may be added to processed foods to compensate for the amino acids deficient in them, e.g. Lysine-enriched bread.

•       Textured vegetable proteins are used successfully to improve the protein quality and reduce the cost of protein-rich foods.

Dietary Sources

•       Present in both plant and animal foods.

•       Animal food sources – highest quality or complete proteins – eggs, milk and milk products – cheese, paneer, mawa, milk powder, curds, condensed milk – meat, fish, shell fish, poultry and organ meats

•       Plant food sources – pulses, especially soya bean (43% protein) and its products; nuts and oilseeds – groundnut and gingelly seeds, cereals (6-12% partially complete proteins); Vegetables except beans and peas are poor sources, green leafy vegetables has approx. (1-3%) and fruit doesn’t have protein content.

Effect of Deficiency and Excess

•       Reduced protein intake – loss of weight, fatigue, anaemia, nutritional oedema, lowered resistance to infection and poor healing of wounds.

•       PCM (Protein Calorie Malnutrition) in pre-school children is kwashiorkar, a deficiency of protein or marasmus, equivalent to starvation in adults.

•       Excess protein results in conversion of fat and stored in adipose tissue in body.

•       Effects kidney and liver as it leads to excrete additional amount of urea.

•       Excess intake leads to high blood levels of cholesterol and increase in loss of calcium through urine.

 

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