Fri. Mar 29th, 2024

 

The large complex molecules referred to as proteins, hold a very vital position in the organisms. They are the prime performer in the cells of the bodies and are therefore needed for framing the structure, performing different functions and regulations of the tissues and organs present in a body. Proteins, the working molecules of a cell, carry out the program of activities encoded by genes. This program requires the coordinated effort of many different types of proteins, which first evolved as rudimentary molecules that facilitated a limited number of chemical reactions.

 Proteins different types are very complicated molecules. With 20 different amino acids that can be arranged in any order to make a polypeptide of up to thousands of amino acids long, their potential for variety is extraordinary. This variety allows proteins to function as exquisitely specific enzymes that compose a cell’s metabolism. An E. coli bacterium, one of the simplest biological organisms, has over 1000 different proteins working at various times to catalyze the necessary reactions to sustain life.

Functions of Protein

Proteins play a pertinent role in initiating, controlling, and regulating various functions that are important for living organisms. Proteins play the following roles:

  • Antibody: As antibodies, the protein binds some specific foreign particles like viruses and bacteria to protect the body from diseases and other abnormalities. So, ultimately it improves the immunity of an organism.
  • Enzyme: As enzymes, proteins carry out thousands of chemical reactions which take place in cells. Different digestive enzymes aid the facilitation of chemical reactions. Also, they act an important role in forming new molecules by reading all the genetic information which is stored in DNA. It aids the expression and regulation of RNA and DNA
  • Messenger: As a messenger, proteins, like hormones, transmit signals for coordinating the different biological processes that take place between tissues and organs.
  • Structural Component: As a structural component, proteins facilitate the structuring and support system for the cells. It also promotes the movement of the body. It acts as a connective agent between different body parts.
  • Transport and storage: Proteins Different Types are responsible for binding and carrying atoms and other small molecules within the cells and also throughout the body.
proteins different types

Structure of Protein

A key to understanding the functional design of proteins is the realization that many have “moving” parts and are capable of transmitting various forces and energy in an orderly fashion. However, several critical and complex cell processes—synthesis of nucleic acids and proteins, signal transduction, and photosynthesis—are carried out by huge genome sequences, researchers can deduce the number and primary structure of the encoded proteins. The term proteome was coined to refer to the entire protein complement of an organism.

For example, the proteome of the yeast Saccharomyces cerevisiae consists of about 6000 different proteins; the human proteome is only about five times as large, comprising about 32,000 different proteins. By comparing protein sequences and structures, scientists can classify many proteins in an organism’s proteome and deduce their functions by homology with proteins of known function.

 Although the three-dimensional structures of relatively few proteins are known, the function of a protein whose structure has not been determined can often be inferred from its interactions with other proteins, from the effects resulting from genetically mutating it, from the biochemistry of the complex to which it belongs, or from all three.

The primary structure of a protein is simply the linear arrangement, or sequence, of the amino acid residues that compose it. Many terms are used to denote the chains formed by the polymerization of amino acids. A short chain of amino acids linked by peptide bonds and having a defined sequence is called a peptide; longer chains are referred to as polypeptides.

Peptides generally contain fewer than 20–30 amino acid residues, whereas polypeptides contain as many as 4000 residues. We generally reserve the term protein for a polypeptide (or for a complex of polypeptides) that has a well-defined three-dimensional structure. It is implied that proteins different types and peptides are the natural products of a cell. The size of a protein or a polypeptide is reported as its mass in daltons (a dalton is 1 atomic mass unit) or as its molecular weight (MW), which is a dimensionless number.

For example, a 10,000-MW protein has a mass of 10,000 daltons (Da), or 10 kilodaltons (kDa). In the last section of this chapter, we will consider different methods for measuring the sizes and other physical characteristics of proteins. The known and predicted proteins encoded by the yeast genome have an average molecular weight of 52,728 and contain, on average, 466 amino acid residues. The average molecular weight of amino acids in proteins is 113, taking into account their average relative abundance. This value can be used to estimate the number of residues in a protein from its molecular weight or, conversely, its molecular weight from the number of residues.

proteins structure

The second level in the hierarchy of protein structure consists of the various spatial arrangements resulting from the folding of localized parts of a polypeptide chain; these arrangements are referred to as secondary structures. A single polypeptide may exhibit multiple types of secondary structure depending on its sequence.

In the absence of stabilizing noncovalent interactions, a polypeptide assumes a random coil structure. However, when stabilizing hydrogen bonds form between certain residues, parts of the backbone fold into one or more well-defined periodic structures: the alpha helix, the beta-sheet, or a short U-shaped turn.

In an average protein, 60 percent of the polypeptide chain exists as helices and sheets; the remainder of the molecule is in random coils and turns. Thus, helices and sheets are the major internal supportive elements in proteins. In this section, we explore forces that favor the formation of secondary structures. The Helix In a polypeptide segment folded into a helix, the carbonyl oxygen atom of each peptide bond is hydrogen-bonded to the amide hydrogen atom of the amino acid four residues toward the C-terminus. This periodic arrangement of bonds confers a directionality on the helix because all the hydrogen-bond donors have the same orientation.

Tertiary structure refers to the overall conformation of a polypeptide chain—that is, the three-dimensional arrangement of all its amino acid residues. In contrast with secondary structures, which are stabilized by hydrogen bonds, tertiary structure is primarily stabilized by hydrophobic interactions between the nonpolar side chains, hydrogen bonds between polar side chains, and peptide bonds. These stabilizing forces hold elements of secondary structure— helices, strands, turns, and random coils—compactly together. Because the stabilizing interactions are weak, however, the tertiary structure of a protein is not rigidly fixed but undergoes continual and minute fluctuation. This variation in structure has important consequences in the function and regulation of proteins.

Proteins Different Types based on structure.

  1. Proteins can be classified on the following basis depending upon shape and structure.
  • Fibrous protein:

Fibrous proteins are elongated or are like fiber. Their axial ratio i.e., the length and breadth ratio is more than 10. They are static and have a simple structure. The biological functions performed by them are less. These are majorly present in animals. These fibrous proteins can further be classified namely, simple fibrous protein and conjugated fibrous proteins.

  • Simple Fibrous Proteins:

Examples include Scleroprotein like Keratine, elastin, fibroin, collagen, etc. are useful for making animal skeletons. These are insoluble in water.

  • Conjugated Fibrous Proteins:

Examples include pigments that are present in the feathers of the chicken.

  • Globular Protein:

Globular proteins are spherical or globular shaped. The axial ratio is mandatorily less than a value of 10. These types of proteins are dynamic i.e., they can move or flow without any restrictions, and the structure is highly complex. These proteins perform diverse biological functions. Main examples of such kinds of proteins include enzymes and hormones. These globular proteins are further classified depending upon composition and solubility. These include:

  1. Homo-globular or simple protein:

Simple proteins are composed of amino acids only. The main examples of simple protein are:

  1. Protamine– These are positively charged proteins with basic nature. They are primarily found in animals. They are water-soluble and also dissolve in ammonium hydroxide solution. They do not coagulate by heat. Also, they precipitate out in alcohol’s aqueous solution.
  2. Histone- These are basic proteins with a weak base as compared to protamine. The molecular weight is less and these are soluble in water. They do not coagulate by heat. These are located in the nucleic acids.
  • Albumin– These are found abundantly in nature but most commonly found in plant seeds and the muscles and blood of animals. The molecular weight of albumin is around 65000 KD. These are soluble in water and can also be coagulated by heat. Examples of plant albumins are Leucosin and Legumelins while examples of animal albumin are serum albumin, lactalbumin, myosin, etc.
  1. Globulin– These include Pseudo-globulin, which is soluble in water, and Euglobulin, which is insoluble in water.
  2. Prolamin– These are a category of storage protein that is found in the seeds. These are majorly insoluble in water but are soluble in dilute acids, detergents, and solution with 60-80% of alcohol. These can be coagulated by heat. Also, prolamines are rich in glutamine and proline.
  1. Conjugate or Hetero-globular or Complex Protein:

Complex proteins are linked by non-protein moiety for becoming functional. So, there are constituted of both, the protein and the non-protein component. The non-protein part is referred to as the prosthetic group. This prosthetic group can be further classified in the following:

  1. Metalloprotein: In metalloprotein, metals like mercury, copper, zinc, silver, etc. strongly bind with proteins like collagen, casein, albumin, etc.
  2. Chromoprotein: This category of protein consists of a colored prosthetic group. For example, Haemoglobin, chlorophyll, peroxidase, myoglobin, etc.
  • Glycoprotein or Mucoprotein: These proteins have a carbohydrate as a prosthetic group. These include an antibody, heparin, hyaluronic acid, etc.
  1. Phosphoprotein: These proteins have phosphate group as prosthetic group. For example, casein, calcineurin, ovovitellin, etc.
  2. Lipoprotein: These proteins have lipid as the prosthetic group. It includes lipovitelline, chylomicrons, etc.
proteins types
  • Derived Protein:

Derived proteins are a derivative of simple or complex proteins that are a result of the actions of heat, chemicals, or enzymes on them. Derived proteins also include some artificially manufactured proteins. These are further sub-divided into primary derived protein and secondary derived protein.

  1. Primary Derived Protein:

The size of these proteins remains unaltered materially but the arrangement is not fixed and is changed. The major examples of proteins different types  are:

  1. Protean-  It is the first by-product obtained after the actions of enzymes, water, or acids on protein. These are insoluble in water. For example – Edestan, myosin.
  2. Metaprotein– These proteins are generated by further actions of different acids or alkalis on protein at a temperature ranging from 30-60 degree-Celcius.
  • Coagulated protein– These proteins are produced by the actions of heat or alcohol on proteins. These are not soluble in water.

Secondary Derived Protein:

These are the types of derived protein in which the size of the original protein is altered. These are manufactured by the actions of digestive enzymes or dilute acids after the hydrolysis has proceeded beyond the level of metaprotein. These proteins are water-soluble and cannot be coagulated by heat. Examples of derived proteins are albumose, globulose, etc.

  1. Categorizing proteins based on the biological functions performed by them.
  • Catalytic protein: Such types of proteins are usually found in enzymes where they help in increasing the rate of metabolism through a catalytic phenomenon. Enzymes secreted by the body have a huge number of proteins, which help in breaking down food and bringing down the food into a form where digestion becomes easier. The enzymes do not take part in digestion but it increases the rate of digestion and hence the catalytic reaction takes place. Enzymes are the biggest example of such types of proteins.
enzymes
  • Structural Protein: Structural protein is the most commonly found protein where the prevalence is mainly among mammals. The structure of the protein is based on collagen fibers, fibronectin, and laminin. This type of protein is used for building cells for various by biotechnologists for cellular attachment. Bones made of collagen, ligaments made of elastin, and keratin made hairs and nails are the biggest examples of structural proteins.
  • Nutrient protein: Nutrient proteins are the basic form of protein required for cell multiplication for bodybuilding and growth. They also serve as a fuel source and provide as much energy as carbohydrates. The simpler form of such proteins is amino acids and they are categorized into 9 major categories which are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. Nutrient protein forms an important part of the body as they are responsible for the structural component of the muscle cells. Hence, they are found in abundance. They are also responsible for the formation of blood cells in the body.
  • Regulatory Protein: The protein which affects the regions of a DNA Molecule having the RNA transcription during the process of transcription is known as regulatory protein.
  • Defensive proteins: The proteins which help us in providing immunity against pathogens and foreign particles are known as defensive proteins. They are responsible for providing antibodies through their generation.
  • Transport protein: Transport nutrients from one organ to the other.
  • Toxic protein: Most proteins are harmful to the body which can get accumulated in cells and can cause harm.
  • Storage protein: They regularize the storage of molecules and ions in cells.
Protein Misfolding

There are many diseases that may occur due to protein misfolding. Protein misfolding is basically the improper folding of proteins that hamper the cells’ health irrespective of the functions performed by the proteins. The major cause of diseases like Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, Creutzfeldt-Jakob disease, cystic fibrosis, Gaucher’s disease, and many other degenerative and neurodegenerative disorders, is supposed to be protein misfolding.

Cellular molecular chaperones, which are ubiquitous, stress-induced proteins, and newly found chemical and pharmacological chaperones have been found to be effective in preventing misfolding of different disease-causing proteins, essentially reducing the severity of several neurodegenerative disorders and many other protein-misfolding diseases.

The role of molecular, chemical, and pharmacological chaperones in suppressing the effect of protein misfolding-induced consequences in humans is explained in detail. Functional aspects of the different types of chaperones suggest their uses as potential therapeutic agents against different types of degenerative diseases, including neurodegenerative disorders.

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