球状タンパク質

WordNet

any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat and eggs and milk and legumes; "a diet high in protein"

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蛋白(たんばく)質
球形の,球状の;小球体(粒)から成る

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/02/20 11:17:46」(JST)

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3-dimensional structure of hemoglobin, a globular protein.

Globular proteins or spheroproteins are spherical ("globe-like") proteins and are one of the common protein types (the others being fibrous, disordered and membrane proteins). Globular proteins are somewhat water-soluble (forming colloids in water), unlike the fibrous or membrane proteins.^[1] There are multiple fold classes of globular proteins, since there are many different architectures that can fold into a roughly spherical shape.

The term globin can refer more specifically to proteins including the globin fold.^[2]

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 ultracentrifuges 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 polar (hydrophilic) amino acids are bound outwards, allowing dipole-dipole interactions 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.

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 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 ligands 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.^[3]

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:

Enzymes, by catalyzing organic reactions taking place in the organism in mild conditions and with a great specificity. Different esterases fulfill this role.
Messengers, by transmitting messages to regulate biological processes. This function is done by hormones, i.e. insulin etc.
Transporters of other molecules through membranes
Stocks of amino acids.
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

Among the most known globular proteins is hemoglobin, a member of the globin protein family. Other globular proteins are the immunoglobulins (IgA, IgD, IgE, IgG and IgM), and alpha, beta and gamma globulins. See protein electrophoresis for more information on the different globulins. Nearly all enzymes with major metabolic functions are globular in shape, as well as many signal transduction proteins.

Albumins are also globular proteins, although, unlike all of the other globular proteins, they are completely soluble in water.

References

^ Andreeva, A (2014). "SCOP2 prototype: a new approach to protein structure mining". Nucleic Acids Res 42: D310–4. doi:10.1093/nar/gkt1242. PMC 3964979. PMID 24293656.
^ Globins at the US National Library of Medicine Medical Subject Headings (MeSH)
^ Travaglini-Allocatelli C; Ivarsson Y; Jemth P; Gianni S (Feb 2009). "Folding and stability of globular proteins and implications for function". Current Opinion in Structural Biology 19 (1): 3–7. doi:10.1016/j.sbi.2008.12.001.

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English Journal

Immunoaffinity Purification of the Glycosylated Extracellular Fragment of Mouse Plexin A2 Produced in a Mammalian Expression System.

Nogi T1, Mihara E2, Yasui N3, Takagi J2.
Methods in molecular biology (Clifton, N.J.).Methods Mol Biol.2017;1493:57-72.
Plexins are type I membrane proteins that function as receptors for semaphorins. All of the known plexins contain a large globular domain, termed the sema domain, in the N-terminal extracellular region, which interacts with semaphorins during signal transduction. Here, we describe procedures for pro
PMID 27787842

Split-Intein Triggered Protein Hydrogels.

Ramirez MA1, Chen Z2,3.
Methods in molecular biology (Clifton, N.J.).Methods Mol Biol.2017;1495:161-171.
Proteins are nature's building blocks and indispensable in living organisms. Protein-based hydrogels have a wide variety of applications in research and biotechnology. In this chapter, we describe an intein-mediated protein hydrogel that utilizes two synthetic soluble protein block copolymers, each
PMID 27714616

Stability of an amphipathic helix-hairpin surfactant peptide in liposomes.

Waring AJ1, Gupta M2, Gordon LM2, Fujii G3, Walther FJ4.
Biochimica et biophysica acta.Biochim Biophys Acta.2016 Dec;1858(12):3113-3119. doi: 10.1016/j.bbamem.2016.09.014. Epub 2016 Sep 21.
Surfactant protein B (SP-B; 79 residues) is a member of the saposin superfamily and plays a pivotal role in lung function. The N- and C-terminal regions of SP-B, cross-linked by two disulfides, were theoretically predicted to fold as charged amphipathic helices, suggesting participation in surfactan
PMID 27664499

Japanese Journal

Disulfide Cross-Linked Network Structure of Intermediate Filament and Matrix in Hair and Wool Cortices

Suzuta Kazuyuki,Arai Kozo
繊維学会誌 71(8), 237-249, 2015
… A two-phase structural model was assumed for the globular matrix keratin-associated protein (KAP) dispersed in the swollen network of intermediate filament (IF) proteins. … The number, type, and location of SS cross-linkages on an IF protein chain were estimated using the results obtained in the reactivity of SS bonds of wool and hair fibers in boiling water and also in the behavior of SS bonds in permanent waving treatment of hair. …
NAID 130005091827

Roles of the highly conserved amino acids in the globular head and stalk region of the Newcastle disease virus HN protein in the membrane fusion process

Sun Chengxi,Wen Hongling,Chen Yuzhen,Chu Fulu,Lin Bin,Ren Guijie,Song Yanyan,Wang Zhiyu
BioScience Trends 9(1), 56-64, 2015
… The hemagglutinin-neuraminidase (HN) protein of NDV has multiple functions including mediating hemadsorption (HAD), neuraminidase (NA), and fusion promotion activities affecting the process of viral attachment, entry, replication and dissemination. … Mutations in the HN globular head and headless HN of NDV were constructed to determinate the impact of highly conserved amino acids in the globular head of paramyxovirus HN proteins and the roles of the stalk region of HN in the fusion process. …
NAID 130005054684

フロリゲン活性化複合体の細胞内イメージング(<特集>ライブセルイメージング)

辻寛之
植物の生長調節 49(2), 112-121, 2014-12-20
… Its molecular nature is a conserved FLOWERING LOCUS T (FT)/ Heading date 3a (Hd3a) protein with small globular structure. … FT/Hd3a protein is synthesized in leaves, transported into shoot apical meristem to initiate floral transition. … Imaging Hd3a proteins and its partner proteins in living cells revealed dynamic process regulating the formation of transcriptional protein complex in the nucleus. …
NAID 110009913070