Volume of substrate affecting enzyme activity

[i_love_science]

Homework Statement

Does enzyme activity increase as the volume of substrate increases?

Relevant Equations

substrates, enzymes

I know that as the concentration of substrate increases, the enzyme activity increases because there are more substrate molecules to react on (but at an increasingly slower rate). Would the same reasoning work with volume? Thanks!

 

[Borek]

Volume of what?

 

[chemisttree]

That would depend on the active site(s). Does the active site # go up with increased volume? I assume you are referring to hydrodymamic volume?

 

[i_love_science]

That would depend on the active site(s). Does the active site # go up with increased volume? I assume you are referring to hydrodymamic volume?

I meant the volume of, for example, a solution of substrate (so the greater the volume, the greater the number of substrates and active sites). If that volume increases, does the reaction rate and enzyme activity increase? If not, could you please share why? Thank you very much.

 

[Borek]

I feel like you are confusing intensive and extensive properties.

Try to think in terms of difference between total activity (volume dependent) and specific activity (volume independent).

 

[i_love_science]

I feel like you are confusing intensive and extensive properties.

Try to think in terms of difference between total activity (volume dependent) and specific activity (volume independent).

Thanks. I'm referring to the specific activity of the enzyme. If there is a greater volume of substrate solution, are there more substrates that can react per enzyme and does the enzyme activity increase?

 

[Borek]

Thanks. I'm referring to the specific activity of the enzyme. If there is a greater volume of substrate solution, are there more substrates that can react per enzyme and does the enzyme activity increase?

Volume of the solution doesn't matter at all, as _specific activity_ is an intensive property.

But your question is still ambiguous, as it is not clear if you refer to the volume of solution in which the reaction takes place (no, it doesn't matter then) or to the volume of reactant used to prepare the final solution - in which case you are changing the final concentrations of reactants, so it has an effect.

 

[i_love_science]

Volume of the solution doesn't matter at all, as _specific activity_ is an intensive property.

But your question is still ambiguous, as it is not clear if you refer to the volume of solution in which the reaction takes place (no, it doesn't matter then) or to the volume of reactant used to prepare the final solution - in which case you are changing the final concentrations of reactants, so it has an effect.

I'm not sure why my question is confusing.

Say, beaker 1 has 5 enzymes and 10 substrate molecules in 10 mL. If the volume of substrate is increased in beaker 2, which still has 5 enzymes, but now has 20 substrate molecules in 20 mL (same concentration, x2 volume), does the enzyme activity change?

 

[Borek]

If the volume of substrate is increased

It is not "volume of substrate", it is "volume of substrate solution".

Now, apply what you know about dilution to calculate new concentrations of both enzyme and substrate and use them to estimate new reaction speed.

 

[i_love_science]

Now, apply what you know about dilution to calculate new concentrations of both enzyme and substrate and use them to estimate new reaction speed.

For beaker 1, enzyme concentration is 0.5 and substrate concentration is 1. For beaker 2, enzyme concentration is 0.25 and substrate concentration is still 1.

The enzyme concentration decreased... so I think the enzyme activity decreased. Is this correct?

Thanks.

 

[epenguin]

For beaker 1, enzyme concentration is 0.5 and substrate concentration is 1. For beaker 2, enzyme concentration is 0.25 and substrate concentration is still 1.

The enzyme concentration decreased... so I think the enzyme activity decreased. Is this correct?

Thanks.

Yes.

You do not give an impression of sound grasp when you talk of concentrations with no units. (But at least you are talking about concentrations now, not volumes vaguely).

Substrate concentration will nearly always be expressed in molar units. Or mM, μM, nM...

Enzyme amount or concentration can be more complicated. Sometimes, when the enzyme is a pure protein, it is expressed as concentration, e.g. ng/ml. But there is no predictable relationship to catalytic activity. So more often you will see in experimental accounts, the amount of enzyme used described in terms of 'enzyme unit' or catalytic unit, I.e. the amount that catalyses a given rate of reaction, e.g. 1 μmole/min under standard conditions.

A catalytic unit that has been recommended is the 'katal'. https://en.wikipedia.org/wiki/Enzyme_unit
I am retired for some time; I've never seen this katal adopted. Why has a unit using minutes rather than seconds been more usual? Because that corresponds to the most usual duration of enzyme assays, so was practically simplifying and direct.

Most usually catalytic rate will be proportional to concentration of enzyme, but not always so it has to be checked experimentallly