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Proteins: Types, Structure, Functions, Deficiency Symptoms and Sources

Updated on December 6, 2018
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Srikanth is passionate about helping people stay healthy. He believes that consuming nutritious food is the key to a long and healthy life.

Proteins Definition

Proteins are nutrients found in food. These nitrogenous organic compounds are large molecules. Proteins are made up of many amino acids. Gerardus Johannes Mulder, a Dutch chemist, was the first person to describe proteins. Protein got its name from Swedish chemist Jöns Jacob Berzelius in 1838.

Proteins Play Many Vital Roles in the Body

Proteins play many vital roles in the body. After water, protein is the most abundant substance in the body. Knowledge about the precise protein composition as well as the functions of individual proteins is essential to understand fundamental processes of cell biology and diseases that are caused by cellular defects.

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Types of Proteins

  • Enzymatic proteins.
  • Antibodies.
  • Structural proteins.
  • Storage proteins.
  • Transport proteins.
  • Contractile proteins.
  • Hormonal proteins.
  • Receptor proteins.


An enzyme is a group of proteins that are made by cells. They act as catalysts in many biochemical reactions. In fact almost all biochemical reactions in the human body need enzymes.


An antibody is a Y-shaped blood protein that is produced by the immune system. Antibodies latch on to foreign substances like bacteria, thereby making them ineffective.

Structural protein

Structural proteins provide internal structure to cells. These large biomolecules are also involved in movement of cells.

Storage protein

Storage proteins are biological reserves of metal ions and amino acids. They are found especially in seeds.

Transport protein

Transport proteins perform the function of moving materials within the body. They bring in ions and other molecules into the cell.

Almonds are Rich in Proteins


Proteins Structure

The structure of protein lays the foundation for its interaction with other molecules in the human body, and, therefore, determines its function.

Primary Structure

Proteins are made up of a long chain of amino acids. Even with a limited number of amino acid monomers – there are only 20 amino acids commonly seen in the human body – they can be arranged in many ways to alter the 3-D structure and function of the protein. The simple sequencing of the protein is known as its primary structure.

Secondary Structure

Secondary protein structure depends on the local interactions between parts of a protein chain, which can affect the folding and 3-D shape of the protein. There are two main things that can alter the secondary structure:

  1. α-helix: N-H groups in the backbone form a hydrogen bond with the C=O group of the amino acid 4 residues earlier in the helix.
  2. β-pleated sheet: N-H groups in the backbone of one strand form hydrogen bonds with C=O groups in the backbone of a fully extended strand next to it.

There can also be many functional groups like alcohols, carboxamines, carboxylic acids, thioesters, thiols, and other basic groups linked to each protein. These functional groups also affect the folding of the proteins and, hence, its function in the body.

Tertiary Structure

Tertiary structure of proteins refers to the overall 3-D shape, after the secondary interactions. These include the influence of polar, nonpolar, acidic, and basic R groups that exist on the protein.

Quaternary Structure

The quaternary protein structure refers to the orientation and arrangement of subunits in proteins with multi-subunits. This is only relevant for proteins with multiple polypeptide chains.

Proteins and Amino Acids

Proteins fold up into specific shapes according to the sequence of amino acids in the polymer, and the protein function is directly related to the resulting 3D structure.

Proteins Interact With Each Other

Proteins may also interact with each other or other macromolecules in the body to create complex assemblies. In these assemblies, they can develop functions that were not possible in the standalone protein, like carrying out DNA replication and the transmission of cell signals.

The nature of proteins is variable. For instance, some proteins are rigid, whereas others are flexible. These characteristics also determine the function of the protein. For instance, rigid proteins may play a role in the structure of the cytoskeleton or connective tissues. On the other hand, flexible proteins may act as hinges, springs, or levers to assist in the function of other proteins.

Stability of Proteins Plays an Important Role in the Evolution of Different Protein Structures

A research study led by Eugene Shakhnovich, a professor of chemistry and chemical biology, and Amy Gilson, a graduate student in Shakhnovich’s lab, suggests that stability of proteins plays an important role in the evolution of different protein structures. Research scientists also found that wide diversity of proteins evolved from a small number of ancestors.

The idea is that very stable proteins are stable for a reason.... If you start to mess with them, that could hurt the organism, so they’re under strong selection to maintain that stability … and that restricts their ability to change their structure.

— Eugene Shakhnovich, Professor of chemistry and chemical biology

A Protein's Function

Proteins prevent many harmful diseases by binding to dangerous foreign particles like bacteria and virus. They execute many chemical reactions in cells. They facilitate the formation of new molecules by reading the genetic information stored in DNA. They act as catalysts.


Proteins Transmit Signals

Proteins transmit signals in order to coordinate various biological processes between cells, tissues and different organs. They provide structure and support for cells. They also transport aroms and molecules within cells and all over the body. Protein acts as a source of energy. It also stores other molecules.

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Symptoms of Protein Deficiency

Proteins are necessary for good health. According to USDA, men need 56 grams and women 46 grams of proteins per day. Here are some known symptoms of protein deficiency:

  • High cholesterol levels.
  • Insomnia.
  • Digestion problems.
  • Weak muscles.
  • Bone pain.
  • Slow healing of wounds.
  • Difficulty in losing weight.
  • Muscle pain.
  • Irregular menstrual cycle.
  • Fatigue.
  • Lack of concentration.
  • Changes in blood sugar that can cause diabetes.
  • Joint pain.
  • Anxiety.
  • Lethargy.
  • Low immunity.


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30 High-Protein Foods

Even though many protein supplements, including protein powder, protein tablets, protein bars, protein biscuits, etc, are available in the market, it is a good idea to get your proteins from the food you eat daily.

There are many proteins in food. If you are wondering how to enhance your protein intake, you can do so by including these foods in your diet:

  1. Navy beans.
  2. Wheat germ.
  3. Edamame.
  4. Swiss cheese.
  5. Tofu.
  6. Peanut butter.
  7. Anchovies.
  8. Greek yogurt.
  9. Mixed nuts.
  10. Soba noodles.
  11. Tilapia
  12. Bean chips.
  13. Cottage cheese.
  14. Sardines.
  15. Dried lentils.
  16. Whey protein.
  17. Quinoa.
  18. Green peas.
  19. Chorizo.
  20. Smoothie drinks.
  21. Roast beef.
  22. Halibut
  23. Milk.
  24. Octopus.
  25. Chicken breast.
  26. Canadian bacon.
  27. Yellowfin tuna.
  28. Light tuna.
  29. Pepperoni.
  30. Jerky.

Younger consumers, especially millennials, are looking for an alternative for red meat, but they don’t want to give up protein.

— Guy Crosby, science editor for America’s Test Kitchen

Dairy Is an Excellent Source of Protein for Children

Dr. Hans H. Stein, professor of animal sciences at the University of Illinois College of Agricultural, Consumer and Environmental Sciences, and his team determined standardized ileal digestibility of crude protein and amino acids in eight sources of animal and plant protein: whey protein isolate, whey protein concentrate, milk protein concentrate, skimmed milk powder, pea protein concentrate, soy protein isolate, soy flour, and whole-grain wheat.

These research scientists derived DIAAS scores from those ileal digestibility values. They also calculated PDCAAS-like scores by applying the total tract digestibility of crude protein in the ingredients to all amino acids.

All dairy proteins tested in the study met Food and Agriculture Organization (FAO) standards as "excellent/high"-quality sources of protein for people six months of age or older, with DIAAS values of 100 or greater.

Relationship Of Proteins With Other Nutrients

Increased protein intake can result in urinary loss of calcium. It is a natural physiological process. Protein reacts with some sugars to produce advanced glycosylation end-products (AGEs) and other compounds (including acrylamides).


  • Proteins are nitrogenous compounds.
  • Proteins are made up of amino acids.
  • Proteins prevent duseases.
  • Proteins provide structure to cells.
  • Women need 46 grams of protein daily.

5 Excellent Sources of Proteins for Vegans

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For a slim, sexy body, it's important to eat protein every day - preferably at every meal. Be sure to ask about the origins of your meat, poultry and seafood. If you can't afford organic, free-range meats, opt for natural poultry, pork, and beef that's raised without antibiotics or hormones.

— Suzanne Somers

© 2017 Srikanth R


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