Globular protein

Globular protein

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Globular proteins, or spheroproteins are one of the two main protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 classes, comprising "globe"
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

-like proteins that are more or less soluble in aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...

s (where they form colloidal solutions). This characteristic distinguishes them from fibrous protein
Fibrous protein
Scleroproteins, or fibrous proteins, constitute one of the three main classes of proteins, alongside globular proteins and conjugated proteins.Keratin, collagen, elastin, and fibroin are all scleroproteins...

s (the other class), which are practically insoluble.

The term globin
Globin
Globins are a related family of proteins, which are thought to share a common ancestor. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members of this family include myoglobin and hemoglobin, which both bind the heme prosthetic group...

 can refer more specifically to proteins including the globin fold
Globin fold
The globin fold is a common three-dimensional fold in proteins. This fold typically consists of eight alpha helices, although some proteins have additional helix extensions at their termini. The globin fold is found in its namesake proteins hemoglobin and myoglobin as well as in phycocyanin proteins...

.

Globular structure and solubility


The term globular protein is quite old (dating probably from the 19th century) and is now somewhat archaic given the hundreds of thousands of proteins and more elegant and descriptive structural motif
Structural motif
In a chain-like biological molecule, such as a protein or nucleic acid, a structural motif is a supersecondary structure, which appears also in a variety of other molecules...

 vocabulary. The globular nature of these proteins can be determined without the means of modern techniques, but only by using ultracentrifuge
Ultracentrifuge
The ultracentrifuge is a centrifuge optimized for spinning a rotor at very high speeds, capable of generating acceleration as high as 2,000,000 g . There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge...

s or dynamic light scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 techniques.

The spherical structure is induced by the protein's tertiary structure
Tertiary structure
In biochemistry and molecular biology, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates.-Relationship to primary structure:...

. The molecule's apolar (hydrophobic) amino acids are bounded towards the molecule's interior whereas polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...

 (hydrophilic) amino acids are bound outwards, allowing dipole-dipole interactions with the solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

, which explains the molecule's solubility.

Globular protein is only marginally stable because the free energy released when the protein folded into its native conformation is relatively small. This is because protein folding requires entropic cost. As a primary sequence of a polypeptide chain can form numerous conformations, native globular structure restricts its conformation to a few only. It results in a decrease in randomness, although non-covalent interactions such as hydrophobic interactions stabilizes the structure.

Although it is still unknown how proteins fold up naturally, new evidence has helped advance understanding. Part of the protein folding problem is that several noncovalent, weak interactions are formed, such as Hydrogen bonds and Van der Waals interactions. Via several techniques, the mechanism of protein folding is currently being studied. Even in the protein’s denatured state, it can be folded into the correct structure. Globular proteins seem to have two mechanisms for protein folding, either the diffusion-collision model or nucleation condensation model, although recent findings have shown globular proteins, such as PTP-BL PDZ2, which fold with characteristic features of both models. These new findings have shown that the transition states of proteins may affect the way that it folds. The folding of globular proteins has also recently been connected to treatment of diseases and anti-cancer ligands have been developed which bind to the folded but not the natural protein. These studies in return have shown that the folding of globular proteins affects its function.

Recall the second law of thermodynamics, the free energy difference between unfolded and folded state is contributed by enthalpy and entropy changes. As the free energy difference in globular protein results from folding into its native conformation is small, it is marginally stable, thus providing a rapid turnover rate and provide effective control of protein degradation and synthesis.

Role


Unlike fibrous proteins which only play a needless function, globular proteins can act as:
  • Enzyme
    Enzyme
    Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

    s, by catalyzing
    Catalysis
    Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

     organic reactions taking place in the organism
    Organism
    In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

     in mild conditions and with a great specificity. Different esterase
    Esterase
    An esterase is a hydrolase enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis.A wide range of different esterases exist that differ in their substrate specificity, their protein structure, and their biological function.- EC classification/list...

    s fulfill this role.
  • Messengers
    Hormone
    A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

    , by transmitting messages to regulate biological processes. This function is done by hormone
    Hormone
    A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

    s, i.e. insulin
    Insulin
    Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....

     etc.
  • Transporter
    Transporter
    Transporter may refer to:* Transporter , types of vehicles designed to transport items* Volkswagen Transporter, a model of van- Film and television :...

    s of other molecules through membrane
    Cell membrane
    The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

    s
  • Stocks of amino acid
    Amino acid
    Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

    s.
  • Regulatory roles are also performed by globular proteins rather than fibrous proteins.

Members


Among the most known globular proteins is hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

, a member of the globin protein family. Other globular proteins are the immunoglobulins (IgA, IgD, IgE, IgG and IgM), and alpha
Alpha globulin
Alpha Globulins are a group of globular proteins in plasma, which are highly mobile in alkaline or electrically charged solutions. They inhibit certain blood protease and inhibitor activity.-Alpha 1 globulins:*α1-antitrypsin*Alpha 1-antichymotrypsin...

, beta
Beta globulins
Beta globulins are a group of globular proteins in plasma thatare more mobile in alkaline or electrically charged solutions than gamma globulins, but less mobile than alpha globulins. Beta globin is on chromosome 11.Examples of beta globulins include:...

 and gamma
Gamma globulin
Gamma globulins are a class of globulins, identified by their position after serum protein electrophoresis. The most significant gamma globulins are immunoglobulins , more commonly known as antibodies, although some Igs are not gamma globulins, and some gamma globulins are not Igs.-Use as medical...

 globulins. See protein electrophoresis
Protein electrophoresis
Protein electrophoresis is a method for analysing the proteins in a fluid or an extract. The electrophoresis may be performed with a small volume of sample in a number of alternative ways with or without a supporting medium: SDS polyacrylamide gel electrophoresis Protein electrophoresis is a method...

 for more information on the different globulins. Nearly all enzymes with major metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 functions are globular in shape, as well as many signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...

proteins.