Protein and Amino-Acids

Protein are complex nitrogenous organic compounds which are essential for physical and mental growth and development of body. They are defined as long chain polymer of α – amino acids bounded by peptide linkage. Therefore, protein are polypeptides of high molecular weight ranging from 5000 to many millions.







Proteins are made by 21 different amino acids. Amino acids are building block of protein, joined together by peptide linkage forming polypeptide.

What is primary source of protein and why?

Plants are primary source of protein because they can synthesize protein by combining nitrogen and water from soil and CO2 from air, while animal cannot make their own protein on their own. Animals depends upon plants to fulfill their protein requirement.

Function of protein in human body.

  1. Body building, growth and renewal of tissue
  2. Synthesis of enzyme, nucleic acid, blood protein and hormones.
  3. Provide energy ( physiological energy value 4 Kcal/g; gross energy value 5.65 Kcal/g)
  4. Provide essential amino acids
  5. Excess protein are converted to fatty acid and glucose.
  6. Essential for nutrient digestion, metabolism and transport of minerals.
  7. Defense mechanism as antibiotics.

Occurrences of protein: Protein occur in animals, plants and microorganisms in significant quantities. However, plants are primary source of protein. In developed countries, people obtain much of their protein form animal source, while in other parts of the world, major portion of dietary protein is derived from cereals and legumes. Many protein are deficient in one or more of the essential amino acids. Therefore, balanced diet is required to make up this deficit.

Single cell protein (SCP): Microorganism contain very large amount of protein. Some selected microorganism have been cultivated in industrial scale to produce commercial protein supplement. Since, the microorganism are single celled entity, the protein thereof is called single cell protein. They make 45% – 85% of their total mass.

Protein content in some food:

Beef17 %Wheat12 %
Pork10 %Pulses22 %
Chicken22 %Soybeans43 %
Fish18 %Peas6.3 %
Milk3.6 %Skimmed milk powder (SMP)36 %
Egg12.5 %Cheese25 %
Rice7 %Fruits2 %

Classification of protein: Although proteins are polypeptides, neutral proteins are in fact very complex in makeup. They are often found complexed with metals, carbohydrates, nucleic acids, lipid etc. conventional system divides protein into three major group.

  1. Simple protein
  2. Conjugated protein
  3. Derived protein

Simple protein: These proteins are made up of amino acids only. For example, albumin, globulin, Glutelin, Prolamin, histones and protamine.  Some important simple protein are

Collagen: They are major portion of white connective tissue (tendon, cartilage, bone). They are not digestible.

Keratin: They are protein of hair, wool, hooves, nail etc.

Albumin: They are found in egg yolk, and whey protein of milk.

Globulin: They are found in meat and soybeans.

Glutelin: these protein occur in cereals such as Glutenin (in wheat), oryzenin (in rice).

Conjugated protein: Conjugated protein are amino acids part combined with non- protein material such as lipid, nucleic acid or carbohydrate. Some of them are;

Chromoprotein:  protein combined with colored compound. Example; hemoglobin, myoglobin, chlorophyll etc.

Glycoprotein: these are the combination of carbohydrate with protein. Example; ovomucin of egg white.

Phosphoprotein: phosphate group are linked to protein group. Example; casein of milk, phosphoprotein of egg yolk.

Lipoprotein: these are the combination of lipid with protein. Example; in milk and egg yolk

Metalloprotein: these are metal and protein complexes. Example; Red blood cells contain Zinc.

Derived protein: These are the compound obtained by chemical or enzymatic modification of native protein. Primary derivatives are slightly modified and insoluble in water. Example rennet coagulated casein. Secondary derivatives are extensively modified, soluble in water and not coagulated by heat. Example includes protease, peptone and peptide.

Amino acids: there are 21 different amino acids that make up the protein. Different kind of protein are found in different kind of food and these protein are made from combination of different amino acid and their grouping.

Essential amino acids: those amino acids that are not synthesized in our body and are required to be include in our diet are called essential amino acids. Out of 21 amino acids, human body can synthesize only 12 amino acids and rest 8 amino acids must be provided from outer source. Essential amino acids are;

Fig: Essential Amino acids



About Author

Name : Pratiksha Shrestha

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.