Storage proteins structure and functions

Storage proteins

Introduction

Storage proteins are a type of protein that acts as a reservoir of metal ions and amino acids, which can be combined and used for maintenance and growth. Proteins are classified according to their structure, origin, and function.

Protein is a type of large organic molecule, consisting of a chain of one or more amino acids. They are the main component of human body cells and the largest unit of cells by dry weight. This macromolecule is usually a chain of 20 amino acids. The human body can make any protein using a combination of these 20 amino acids (1) & (2).

Storage protein

It is a type of protein that is stored inside cells or tissues as food and can be assembled when needed to provide energy. This type of protein stores amino acids for the body until it is ready for use. Many storage proteins store amino acids in embryos and developing organisms.

These types of proteins are stored in a special structure called the protein body. It is located in the cotyledons and endosperm of the seed. Enzymes are needed to catabolize these storage proteins into amino acids. These enzymes are stored in dried seeds. However, various storage proteins are synthesized as new enzymes after imbibition (2).

Example

Storage proteins are mainly present in plant seeds, egg whites, and milk. There is also another type of storage protein. This is ferritin, which stores iron.

Structure

All storage proteins contain sulfur in the form of the amino acid cysteine and methionine. Various organisms are able to adapt to changes in the relative availability of nitrogen and sulfur to specific types of storage proteins that consist of high and low sulfur amino acids. Some storage proteins contain only amino acids.

This type of protein can be found in all plant and animal bodies. The proteins present in plant seeds are mainly storage proteins. Basically plant seeds consist of high concentrations of storage proteins. About 25% of the dry weight of plant seeds is made up of storage proteins.

There are two types of storage protein present in an animal’s body. These are ovalbumin and casein. They also play a vital role in an animal’s body. All storage proteins basically integrate directly into the endoplasmic reticulum as embryonic and endosperm terminally isolated cell protein bodies and protein storage vacuoles.

Types of storage proteins

Animal storage protein

1. Ovalbumin

  • Ovalbumin is mainly a glycoprotein. It is amply large and complex to be mildly immunogenic.
  • They are found in the white part of an egg.
  • It is a monomeric phosphoglyco-protein.
  • Ovalbumin protein consists of 385 amino acids.
  • Its molecular mass is about 42.7 kDa.
  • Egg white contains about 55% of ovalbumin.
  • It takes on a serpin-type structure.
  • Ovalbumin contains N-terminal acetylation, phosphorylation, and glycosylation.
  • It is only the egg white protein that contains free sulfhydryl groups (4).

2. Casein

  • Casein is a storage protein found in large amounts (about 80%) in mammalian milk.
  • Pure casein is a shapeless white solid without taste. And commercial casein is slightly yellow, with a pleasant odor.
  • These proteins ensure a long and uniform supply of essential amino acids in the body.
  • Casein tops the list of substances that contribute to weight loss without harming the body.
  • It reduces catabolism and the amount of fat in the body.
  • Casein protein is taken as a shake, mixed with milk or juice.
  • It makes up about 40% of the protein obtained from cow’s milk.
  • These proteins are slowly digesting milk protein.
  • It slowly releases amino acids into the human bloodstream.
  • Casein is hydrophobic and less soluble in water.
  • Heat does not cause casein to accumulate or become indigestible (3).

3. Ferritin

  • Ferritin is an important protein that stores iron inside human cells.
  • It is a globular protein complex having a spherical structure of 48,000 MW.
  • Its molar mass is 474 kDa.
  • They are found in the cytoplasm, mitochondria, and nucleus.
  • This type of protein is found in various tissues as a cytosolic protein.
  • Ferritin is the main iron-storage protein in both eukaryotic and prokaryotic cells in its ferric state.
  • Consist of 24 protein subunits forming a hollo nanocage with multiple metal protein interactions.
  • It is able to store and release iron in a controlled manner.
  • Liver, chicken, tofu, lentils, and beans are good sources of ferritin.
  • Cereals and seeds contain ferritin as the main source of iron. The presence of ferritin in grains and seeds proves that it is a long-term iron storage protein.
  • This protein can carry up to 4000 iron molecules (5).

 Plant storage proteins

Plant storage proteins are mainly two types. These are seed and vegetative storage proteins.

1. Seed storage proteins

  • They are a type of protein that store at high levels in the seed at the last stage of seed development.
  • When seeds germinate, seed storage proteins are depleted and as a result, amino acids are used by developing seedlings as a source of nutrients.
  • These are the primary proteins of all types of grains.
  • They illustrate plant proteins that are eaten in large quantities by humans.
  • These proteins are built from a number of different polypeptide chains.
  • They are non-enzymatic (2).

2. Vegetative storage proteins

  • Vegetative storage proteins are proteins that collect in plant tissues such as leaves, stems, etc.
  • They are collected in plant tissues when additional resources are available.
  • Such proteins act as a temporary store of amino acids for use in the stage of growth and development of plants.
  • These types’ proteins are found in soybean, potato, sweet potato, etc. (1) & (2).

Function or uses of storage proteins

1. The primary function of seed storage protein is to act as a storage for nitrogen, carbon, and sulfur. These proteins move rapidly during seed germination and act as an essential source of nitrogen depletion for plants.

2. Ovalbumin provides the developing organism with a ready source of amino acids and organic nitrogen (4).

3. Casein is rich in amino acids which act as building blocks for producing new muscle or energy.

4. Ovalbumin is an antigen. It can be used to remove cystitis by injection into the bladder.

5. Casein is used as a nutritional supplement. This protein is anti-catabolic. It slows down the rate of digestion and reduces the breakdown of proteins in the body due to the constant supply of amino acids to the muscle cells.

6. The main function of ferritin in the human body is to act as a buffer against iron deficiency and iron overload.

7. Ferritin proteins store iron in a non-toxic form and transport it where needed. They protect all cells from its toxic reaction (2) & (4).

Q&A

1. What are storage proteins? What do storage proteins do?

They are a type of protein that act as biological storage of metallic ions and amino acids used by organisms. These are mainly found in plant seeds, milk, and egg white.

Function

  • Stored in both plant and reproductive tissues.
  • They act as a reservoir for use in plant’s growth and development.
  • These types of protein protect the cells.
  • It acts as a building block for producing new muscle.

2. How do storage proteins work?

  • It works as biological reserves of metal ions and amino acids.
  • They store iron which is a component of heme.
  • It is a primary source of dietary plant proteins.
  • Plant storage proteins act as an essential source of nitrogen depletion for plants.
  • These types of protein play an important role in the formation of seed germination and plant growth.

3. Proteins are considered storage molecules for which of the following elements?

For Nucleic acid

4. Which hormone causes a decrease in fat storage and an increase in the synthesis of muscle proteins?

The hormone that causes a decrease in fat storage and an increase in the synthesis of muscle proteins is growth hormone.

Written By: Manisha Bharati

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