Editing Globular protein

{{Short description|Spherical, water-soluble type of protein}}
{{More citations needed|date=July 2007}}
[[Image:1GZX Haemoglobin.png|thumb|300px|3-dimensional structure of [[hemoglobin]], a globular protein.]]

In [[biochemistry]], '''globular proteins''' or '''spheroproteins''' are spherical ("globe-like") [[protein]]s and are one of the common protein [[Protein fold class|types]] (the others being [[fibrous protein|fibrous]], [[disordered protein|disordered]] and [[membrane protein]]s). Globular proteins are somewhat water-soluble (forming [[colloid]]s in water), unlike the fibrous or membrane proteins.<ref>{{cite journal | vauthors = Andreeva A, Howorth D, Chothia C, Kulesha E, Murzin AG | title = SCOP2 prototype: a new approach to protein structure mining | journal = Nucleic Acids Research | volume = 42 | issue = Database issue | pages = D310-4 | date = January 2014 | pmid = 24293656 | pmc = 3964979 | doi = 10.1093/nar/gkt1242 }}</ref> There are multiple [[Protein fold class|fold classes]] of globular proteins, since there are many different architectures that can [[Protein folding|fold]] into a roughly spherical shape.

The term [[globin]] can refer more specifically to proteins including the [[globin fold]].<ref>{{MeshName|Globins}}</ref>

==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]] vocabulary. The globular nature of these proteins can be determined without the means of modern techniques, but only by using [[ultracentrifuge]]s or dynamic light [[scattering]] techniques.

The spherical structure is induced by the protein's [[tertiary structure]]. The molecule's [[apolar]] (hydrophobic) amino acids are bounded towards the molecule's interior whereas [[Chemical polarity|polar]] (hydrophilic) amino acids are bound outwards, allowing [[dipole–dipole interaction]]s with the [[solvent]], which explains the molecule's solubility.

Globular proteins are 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 stabilize the structure.

===Protein folding===
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 non-covalent, weak interactions are formed, such as hydrogen bonds and [[Van der Waals force|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, that fold with characteristic features of both models.  These new findings have shown that the transition states of proteins may affect the way they fold. The folding of globular proteins has also recently been connected to treatment of diseases, and anti-cancer [[ligand]]s have been developed which bind to the folded but not the natural protein.  These studies have shown that the folding of globular proteins affects its function.<ref>{{cite journal | vauthors = Travaglini-Allocatelli C, Ivarsson Y, Jemth P, Gianni S | title = Folding and stability of globular proteins and implications for function | journal = Current Opinion in Structural Biology | volume = 19 | issue = 1 | pages = 3–7 | date = February 2009 | pmid = 19157852 | doi = 10.1016/j.sbi.2008.12.001 }}</ref>

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

==Role==
Unlike fibrous proteins which only play a structural function, globular proteins can act as:
* [[Enzyme]]s, by [[catalysis|catalyzing]] organic reactions taking place in the [[organism]] in mild conditions and with a great specificity. Different [[esterase]]s fulfill this role.
* [[Hormone|Messengers]], by transmitting messages to regulate biological processes. This function is done by [[hormone]]s, i.e. [[insulin]] etc.
* [[Membrane transport protein|Transporters]] of other molecules through [[cell membrane|membrane]]s
* Stocks of [[amino acid]]s.
* [[Regulatory]] roles are also performed by globular proteins rather than fibrous proteins.
* Structural proteins, e.g., [[actin]] and [[tubulin]], which are globular and soluble as monomers, but polymerize to form long, stiff fibers

==Members==
{{unreferenced section|date=March 2018}}
Among the most known globular proteins is [[hemoglobin]], a member of the [[globin protein family]]. Other globular proteins are the [[Alpha globulin|alpha]], [[Beta globulin|beta]] and [[Gamma globulin|gamma]] (IgA, IgD, IgE, IgG and IgM) globulin. See [[protein electrophoresis]] for more information on the different globulins. Nearly all enzymes with major [[metabolism|metabolic]] functions are globular in shape, as well as many [[signal transduction]] proteins.

[[Albumin]]s are also globular proteins, although, unlike all of the other globular proteins, they are completely soluble in water. They are not soluble in oil.

==References==
{{Reflist}}

{{Protein topics}}

[[Category:Proteins by structure]]
[[Category:Protein structure]]