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Proteins are complex molecules involved in various cellular activities. Proteins are made up of various amino acids. There are twenty different types of amino acids. In proteins, amino acids are linked through peptide bonds.
Amino acids are monomeric units of proteins and enzymes. Amino acids are linked one another through peptide bonds. All natural amino acids are L-α-amino acids except glycine. So, all peptides and proteins also made up of L-α-amino acids. The 20 amino acids are:
alanine, arginine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, valine.
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Essential Amino acids
Some amino acids are synthesized in the body. But others are not synthesized in the body and must be incorporated in diet. These are called essential amino acids. There are total of nine essential amino acids.
They are isoleucine, leucine, lysine, threonine, tryptophan, methionine, histidine, valine and phenylalanine. Histidine is essential only for children. Mnemonics used for remembering essential amino acids I Like Licking Toads That Make Hallucination Very Possible
Different types of proteins are present in our body and each one performs specific function. Skeletal muscle contains proteins called actin and myosin. They involve in muscle movement. Enzymes catalyze metabolic activities in a cell. Enzymes are also made up of proteins. The globin part of hemoglobin that involve in transport of oxygen is made up of protein. Iron is stored in the liver as a complex molecule with protein called ferritin. Bones contain protein called collagen. In the plasma of the blood we have different types of protein molecules like albumins, globulins that fight against diseases. Antibodies are made up of proteins. Some proteins are hormones and they involve in cellular growth and differentiation.
Protein less diet could be dangerous. Malnutrition associated with less protein intake could lead to growth failure, weakened skeletal and cardiac muscles. And also less protein can lead to decreased immunity that encourages many diseases to attack. Less protein diet leads to kwashiorkor syndrome that usually affects children. Swollen abdomen, discoloration of hair and skin are the symptoms.
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Poultry, fish, sea food, milk, cheese, yogurt, egg, beans, soya, nuts are the good sources of proteins. Some of the protein containing foods provides complete proteins. That means they contain all types of amino acids that are needed to build proteins. Meat, fish and dairy products mainly provide complete proteins. In other hand, some foods contain only certain types of proteins. They are called partial proteins. Vegetables, fruits, beans, grains and nuts provide only certain types of amino acids. So, vegetarians especially need to take different types of food. Unlike carbohydrates, there are no bad proteins. But the other macromolecules come along with, makes the difference. For example, egg white contains proteins and no cholesterol. But if we eat egg yolk, we consume approximately 300 mg of cholesterol. Fish is the good source of proteins. Though it contains fat it is still good. Because, fish contains omega-3 fatty acids. Omega-3 fatty acid is good for heart and hair. Skimmed milk and dairy products are good source of protein and calcium. Beans are the good source of proteins as well as fiber. Taking soya in small quantities do help in heart disease. However taking excess of soyamilk could change hormone levels. Because soya contain phytohormones.
Proteins are polymers of amino acids. Amino acids are linked together with peptide bonds. The peptide bonds between amino acids are amide bonds.
Proteins are classified into
Simple proteins e.g. albumin, globulin, glutelin. Simple proteins upon hydrolysis yield α- amino acids. Glutelin is a protein present in wheat.
Conjugated proteins e.g. these are classified based on prosthetic group. Prosthetic group is a non protein component of a conjugated protein.
Mucin( Glycoprotein)
Casein (Phosphoprotein)
Nuclein ( Nucleic acid)
Lipoportein (Lecithin)
Derived proteins e.g. metaproteins, peptones. These are obtained after degradation by acid, alkali or enzymes.
Proteins also classified as fibrous proteins e.g. collagen, fibroin (silk) and Globular proteins (which are soluble in water and dilute acids and alkali).
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Approximately fifty percent of ingested proteins are utilized for making enzymes and rest for making nucleic acids, neurotransmitters and cell structure. The enzymes involved in protein digestion are called proteases. Protein digestion first begins in stomach. In the stomach, parietal cells produce hydrochloric acid and chief cells produce pepsinogen. In the presence of hydrochloric acid, pepsinogen is then converted into pepsin. Proteins are broken down to peptides, proteoses and peptones by hydrolysis and hydrochloric acid enhances the breakdown of peptide bonds. In the duodenum (upper part of small intestine) pancreatic enzyme, trypsin and chymotripsin also involves in protein digestion and converts to amino acids. Pancreatic carboxypeptidases, elastage, collaganase, amino peptidases also involve in breakdown of peptides to amino acids. Amino acids are absorbed through small intestine blood and then to liver. From the liver they are transported through blood stream to the different cells of the body. Within the cells amino acids continuously combine to form proteins and breakdown to amino acids.
The daily intake of proteins depends on person to person and on muscle mass. Some may need more and some need less. However, daily diet should not contain more than 30 percent. It also depends on the total calories you need as proteins also provide certain amount of calories. If you are doing any resistant exercises you may need more protein. Consult your doctor to know how much you need.
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When we consume excess protein, it is converted into sugars or fatty acids. Excess protein digestion causes more acid liberation and to neutralize the excess acid, calcium is involved. This causes bones to weaken. Excess protein provides excess amino acids. The liver removes nitrogen from amino acids in the form of urea during catabolism. Normally nitrogen participates in urea formation that is excreted by the kidneys. Kidneys can eliminate certain amounts only. In case of kidney disease, the amount will be much lesser. Accumulation of nitrogen and urea causes disturbance in nitrogen balance in the blood and also causes kidney damage.
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