I am trying to answer the question below, and I am guessing the reason lactate has a lower Km is that it is better able to accept electrons than ethanol. Is this correct? If so, what makes lactate a better electron acceptor? Any help is greatly appreciated!
"ethanol is a very poor substrate for LDH
Would you expect the Km for ethanol to be higher or lower than that for lactate? Justify your answer with reference to the active site structure and the structures of lactate and ethanol. Include sketches of the structures of lactate and ethanol at pH 7.0"
To answer whether the Km for ethanol would be higher or lower than that for lactate, we need to consider the active site structure and the structures of lactate and ethanol.
The Km value represents the substrate concentration required for an enzyme to reach half its maximum velocity, and it can provide insights into the affinity of the enzyme for its substrate. A lower Km indicates higher affinity, meaning the enzyme can bind to the substrate more easily.
In the case of lactate dehydrogenase (LDH), lactate is a better substrate than ethanol. The reason for this lies in the difference in structures between lactate and ethanol and their compatibility with the active site of LDH.
Lactate is a negatively charged molecule, specifically a carboxylate ion (CH3CH(OH)COO−). This charge allows it to interact favorably with positively charged residues within the active site of LDH. These interactions stabilize the binding of lactate to the enzyme, resulting in a lower Km value. Additionally, lactate possesses a hydroxy group (OH) that can participate in hydrogen bonding with the active site residues, further enhancing its binding.
On the other hand, ethanol (CH3CH2OH) is a neutral molecule without a negative charge. It lacks the ability to establish strong ionic interactions within the active site of LDH. As a result, its binding to the enzyme is weaker, leading to a higher Km value.
It is important to note that the active site structure of LDH features specific amino acid residues that can interact with lactate and participate in catalysis. The exact details of the active site structure can vary depending on the specific isozyme of LDH being considered (e.g., LDH-A or LDH-B), but the general principles discussed here remain applicable.
To visualize the structures of lactate and ethanol at pH 7.0, you can refer to chemical structure databases, organic chemistry textbooks, or chemical drawing software. These resources often provide 2D structures or molecular models that represent the structures of different compounds.
By considering the differences in the structures of lactate and ethanol and their interactions with the active site of LDH, we can conclude that lactate will likely have a lower Km compared to ethanol.