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==Structural Overview== | ==Structural Overview== | ||
The size of hexokinase type I is approximately 100 kD.<ref>Fromm H., Zewe V. Kinetic studies of the brain hexokinase reaction. The Journal of Biological Chemistry. 1962.'''235''':1661-1667.</ref> Hexokinase type I is constructed by a N-terminal regulatory domain and a C-terminal catalytic domain joined together by an [http://en.wikipedia.org.wiki/alpha_helix alpha helix]. The glucose binding site of hexokinase type I can be found within the two sub-units that make up the isoenzyme, these are known as lobes. Factors that contribute to the binding of glucose to this active site include amino acids within the actual site and [http://en.wikipedia.orh.wiki/hydrogen_bong hydrogen bonding] that takes place on the glucose between the hydroxyl groups.<ref>Bianchi M., Bolognesi M., Deriu D., Magnani M., Murshudov G., Rizzi M., Rosano C., Sabini E., Serafini G. Binding of non-catalytic ATP to human hexokinase I highlights the structural components for enzyme-membrane association control. Structure. 1999. '''11''': 1427-1437. PMID [http://www.ncbi.nlm.nih.gov/pubmed/10574795?dopt=Abstract 10574795]</ref> | The size of hexokinase type I is approximately 100 kD.<ref>Fromm H., Zewe V. Kinetic studies of the brain hexokinase reaction. The Journal of Biological Chemistry. 1962.'''235''':1661-1667.</ref> Hexokinase type I is constructed by a N-terminal regulatory domain and a C-terminal catalytic domain joined together by an [http://en.wikipedia.org.wiki/alpha_helix alpha helix]. The glucose binding site of hexokinase type I can be found within the two sub-units that make up the isoenzyme, these are known as lobes. Factors that contribute to the binding of glucose to this active site include amino acids within the actual site and [http://en.wikipedia.orh.wiki/hydrogen_bong hydrogen bonding] that takes place on the glucose between the hydroxyl groups.<ref>Bianchi M., Bolognesi M., Deriu D., Magnani M., Murshudov G., Rizzi M., Rosano C., Sabini E., Serafini G. Binding of non-catalytic ATP to human hexokinase I highlights the structural components for enzyme-membrane association control. Structure. 1999. '''11''': 1427-1437. PMID [http://www.ncbi.nlm.nih.gov/pubmed/10574795?dopt=Abstract 10574795]</ref> | ||
[[Image:Hexlobes.jpg|thumb|right|The C-terminal and N-terminal regions of Hexokinase Type I - held together by hydrogen bonding and a connecting helix.<ref>Hay N., Robey R. Mitochondrial hexokinases, novel mediators of the antiapoptotic effects of growth factors and Akt. Oncogene. 2006. '''25''': 4683-4696.</ref>]] | [[Image:Hexlobes.jpg|thumb|right|'''Figure 2:'''The C-terminal and N-terminal regions of Hexokinase Type I - held together by hydrogen bonding and a connecting helix.<ref>Hay N., Robey R. Mitochondrial hexokinases, novel mediators of the antiapoptotic effects of growth factors and Akt. Oncogene. 2006. '''25''': 4683-4696.</ref>]] | ||
===Glucose Binding Sites=== | ===Glucose Binding Sites=== | ||
The residues of the glucose binding site of Hexokinase Type I are very highly conserved within the hexokinase sequence; glucose binds equally to both domains or "lobes" of the structure. As a result, hexokinase type I in its native conformation has an active site in its inactive regulatory domains. Before glucose binds to hexokinase type I, it is said to be in an open conformation. ATP is already bound within one of the domains but it is situated a distance from the glucose binding site. As the glucose binds, the two domains close around the glucose substrate and this change results in a newly formed pattern of hydrogen bonding.<ref name="one" /> | The residues of the glucose binding site of Hexokinase Type I are very highly conserved within the hexokinase sequence; glucose binds equally to both domains or "lobes" of the structure. As a result, hexokinase type I in its native conformation has an active site in its inactive regulatory domains. Before glucose binds to hexokinase type I, it is said to be in an open conformation. ATP is already bound within one of the domains but it is situated a distance from the glucose binding site. As the glucose binds, the two domains close around the glucose substrate and this change results in a newly formed pattern of hydrogen bonding.<ref name="one" /> | ||
[[Image:Glucosebinding.jpg|thumb|left|The conformational change in hexokinase caused by glucose binding.<ref>Martin D., Huang P., Pelicano H., Xu R. Glycolysis inhibition for anticancer treatment. Oncogene. 2006. '''25''': 4633-4646.</ref>]] | [[Image:Glucosebinding.jpg|thumb|left|'''Figure 3:'''The conformational change in hexokinase caused by glucose binding.<ref>Martin D., Huang P., Pelicano H., Xu R. Glycolysis inhibition for anticancer treatment. Oncogene. 2006. '''25''': 4633-4646.</ref>]] | ||
===Active Site=== | ===Active Site=== | ||
In the <scene name='Sandbox_172/Mynewscene/2'>active site</scene> of hexokinase type I, <scene name='Sandbox_172/Residue1/4'>Lys 621</scene>-denoted in red, and <scene name='Sandbox_172/Residue1/5'>Asp 657</scene>-denoted in blue, show the hydrogen-bonding distance in which a reactive O6 hydroxyl is situated in between. Here Lys 621 functions to aid the transfer of the phosphate moiety which is negatively charged; Asp 657 serves as a catalytic base or to position the glucose O6 correctly for phosphoryl transfer to take place.<ref name="one" /> Though it may seem that | In the <scene name='Sandbox_172/Mynewscene/2'>active site</scene> of hexokinase type I, <scene name='Sandbox_172/Residue1/4'>Lys 621</scene>-denoted in red, and <scene name='Sandbox_172/Residue1/5'>Asp 657</scene>-denoted in blue, show the hydrogen-bonding distance in which a reactive O6 hydroxyl is situated in between. Here Lys 621 functions to aid the transfer of the phosphate moiety which is negatively charged; Asp 657 serves as a catalytic base or to position the glucose O6 correctly for phosphoryl transfer to take place.<ref name="one" /> Though it may seem that | ||
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==Functional Overview== | ==Functional Overview== | ||
===Basic Function=== | ===Basic Function=== | ||
Hexokinase Type I functions in a mainly catabolic role; it is responsible for introducing glucose the glycolytic process in attempts to produce ATP. Hexokinase Type I phosphorylates a hexose into a hexose phosphate; most commonly the substrate of hexokinase I is found to be glucose and the product found to be glucose-6-phosphate.<ref>Bianchi M., Casabianca A., Magnani M., Serafini G., Stocchi V. Human hexokinase type I microheterogeneity is due to different amino-terminal sequences. The Journal of Biological Chemistry. 1991. '''266''': 502-505.</ref> Mammalian brain tissue shows a high content of Hexokinase Type I which reiterates the idea that this isoenzyme is needed to maintain high rates of energy metabolism. Hexokinase Type I associated with brain homogenates | Hexokinase Type I functions in a mainly catabolic role; it is responsible for introducing glucose the glycolytic process in attempts to produce ATP. Hexokinase Type I phosphorylates a hexose into a hexose phosphate; most commonly the substrate of hexokinase I is found to be glucose and the product found to be glucose-6-phosphate.<ref>Bianchi M., Casabianca A., Magnani M., Serafini G., Stocchi V. Human hexokinase type I microheterogeneity is due to different amino-terminal sequences. The Journal of Biological Chemistry. 1991. '''266''': 502-505.</ref> Mammalian brain tissue shows a high content of Hexokinase Type I which reiterates the idea that this isoenzyme is needed to maintain high rates of energy metabolism. Hexokinase Type I associated with brain homogenates demonstrates an interaction with outer mitochondrial membrane. The binding of to this mitochondria is highly dependent on the N-terminus sequence. The protein porin is then responsible for the formation of a channel in which metabolites can pass through the mitochondrial membrame.<ref>Wilson, J. Isozymes of mammalian hexokinase: structure, subcellular localization and metabolic function. The Journal of Experimental Biology. 2003.'''206''':2049-2057.</ref> | ||
===Allosteric Regulation=== | ===Allosteric Regulation=== | ||
Entry of glucose into a cell is highly dependent on the number of glucose transporters present on the cell surface and the affinity that these specific transporters have for glucose. Expression of these glucose transporter members vary in level and strength different from tissue to tissue. Hexokinase type I has a low Km and therefore a high affinity for glucose. This allows the initiation of glycolysis even when blood glucose levels are relatively low. The inhibition of hexokinase type I is caused by its product, glucose-6-phosphate. This inhibitory step prevents over-consumption of cellular ATP when glucose is not limiting. <ref>Garavito R., Mulichak A., Sebastian S., Wilson J. Allosteric Regulation of Type I Hexokinase: A Site-Directed Mutational Study Indicating Location of the Functional Glucose-6-Phosphate Binding Site in the N-terminal Half of the Enzyme. Archives of Biochemistry and Biophysics. 1999. '''15''': 203-210.</ref> | |||
===Mechanism of Inhibition and Relief=== | |||
The activity of hexokinase type I becomes inihibited when glucose-6-phosphate binds to the inactive, N-terminal half of the enzyme. This binding stimulates the inhibition of the active, C-terminal half and as a result, the catalysis of glycolysis becomes stagnant. The presence of orthophosphate relieves the product inhibition by displacing the bound glucose-6-phosphate and binding to the enzyme itself. The N-terminal half then responds differently to the newly bound orthophosphate and evokes an active reaction from the C-terminal half. The ability of the C-terminal half to significantly differentiate between the two possible binding molecules is evident by the presence of ADP-ligated active sites on the hexokinase type I enzyme as well as ADP binding sites on the N-terminal halves.<ref name="two" /> | |||
==References== | ==References== | ||
<references /> | <references /> | ||
<table style="background-color:#ffffc0" cellpadding="8" width="95%" border="0"><tr><td>Please do NOT make changes to this Sandbox until after April 23, 2010. Sandboxes 151-200 are reserved until then for use by the Chemistry 307 class at UNBC taught by Prof. [[User:Andrea Gorrell|Andrea Gorrell]].</td></tr> | <table style="background-color:#ffffc0" cellpadding="8" width="95%" border="0"><tr><td>Please do NOT make changes to this Sandbox until after April 23, 2010. Sandboxes 151-200 are reserved until then for use by the Chemistry 307 class at UNBC taught by Prof. [[User:Andrea Gorrell|Andrea Gorrell]].</td></tr> | ||
