Serine hydroxymethyltransferase: Difference between revisions
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The figure on the left represents the substrate binding interactions with specific residues of the SHMT enzyme. T342 and N116 are very important residues for substrate binding within PLP. Once again the polar residues of SHMT bind to the polar serine substrate. A key factor for both the PLP and substrate binding is the specific hydrogen donating/accepting by the respective parts. One possible hypothesis to inhibition of SHMT comes from the differences in hydrogen bonding between substrate and inhibitor. In the substrates case, the serine substrate is primarily proton donating. In the case of inhibition through the use of antifolates, there are more hydrogen accepting atoms than hydrogen donating atoms. In fact, the binding portion of tetrahydrofolate (substrate) compared to methotrexate (inhibitor) makes it clear. THF substrate contains three proton donating atoms within the binding portion while methotrexate contains three proton accepting atoms within the binding portion. As we know, hydrogen bonding stabilizes the secondary structure of a protein. When methotrexate is introduced, hydrogen bonding shifts and destabilizes the protein. | The figure on the left represents the substrate binding interactions with specific residues of the SHMT enzyme. T342 and N116 are very important residues for substrate binding within PLP. Once again the polar residues of SHMT bind to the polar serine substrate. A key factor for both the PLP and substrate binding is the specific hydrogen donating/accepting by the respective parts. One possible hypothesis to inhibition of SHMT comes from the differences in hydrogen bonding between substrate and inhibitor. In the substrates case, the serine substrate is primarily proton donating. In the case of inhibition through the use of antifolates, there are more hydrogen accepting atoms than hydrogen donating atoms. In fact, the binding portion of tetrahydrofolate (substrate) compared to methotrexate (inhibitor) makes it clear. THF substrate contains three proton donating atoms within the binding portion while methotrexate contains three proton accepting atoms within the binding portion. As we know, hydrogen bonding stabilizes the secondary structure of a protein. When methotrexate is introduced, hydrogen bonding shifts and destabilizes the protein. | ||
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=Mechanism of Action= | =Mechanism of Action= | ||
[[Image:S_to_G_Mechanism.png|right|thumb|400px| Mechanistic Conversion of Serine to Glycine catalyzed by SHMT]] | |||
The most common accepted mechanism of action is through a process of modified retro-aldol cleavage. In this case, the mechanism starts with the serine/PLP-enzyme complex. The base represented by 'B' attacks the hydrogen of the ethyl alcohol functional group. The resonance allows the process of tautomerization to occur. This end results of tautomerization results in the formation of a formaldehyde, and acidic 'H-A' compound, and an imine attached to the serine-PLP complex. The double bond of the imine then reacts with the acidic 'H-A' compound. Resonance then allows the new complex to form the separated glycine and PLP-enzyme. In the case of serine hydroxymethyltransferase, the pyridoxal-5'-phosphate is the cofactor. The SHMT enzyme has no enzymatic activity without the presence of this cofactor. On the large C-terminal domain, PLP is covalently bound to the N-terminus of an alpha helix following a beta strand. The K226 residue specifically is important for the binding. The amino group on this lysine residue helps bind PLP. The phosphate group of PLP is bound to the N-terminus of the next alpha helix. The PLP will pack against the B strands present. The binding of PLP to SHMT supports conformation of the active site which is located in a space between the two monomeric domains at the border between the two monomeric units of the SHMT dimer. For now, serine and tetrahydrofolate (THF) are the only known biological substrates of SHMT. The end products are of course glycine and 5,10-methylene tetrahydrofolate. If the reaction is flipped, the substrates and products are also flipped. An important point of the SHMT mechanism is that fact that the reaction can start with either serine or glycine as the substrate. As far as inhibitors, antifolates are known to reduce and/or eliminate the enzymatic activity of SHMT. This is not surprising because of the fact that folate compounds are substrates of SHMT. | |||
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