Uncompetitive Inhibition and Noncompetitive Inhibition are two separate entities. Noncompetitive Inhibition is a special case of mixed inhibition. Cue brief, but adequately detailed summary
To start, the equation for the initial velocity of a Michaelis-Menten enzyme can be given by the equation:
Vo = Vmax/
bKm + c(S)
Where b and c are both constants, specific for a given inhibitor/enzyme combo. We can use this equation for competitive, uncompetitive, and mixed inhibiton. (S) is the substrate concentration, Vmax is the maximum velocity the particular enzyme-catalyzed reaction can attain at very high substrate concentrations, Vo is the initial velocity of the reaction, and Km is the Michaelis constant.
An
uncompetitive inhibitor will bind a site distinct from the active site of the enzyme (opposed to a
competitive inhibitor which competes with the substrate for the active site) and will bind only to the enzyme-substrate (ES) complex.
In the presence of an
uncompetitive inhibitor the typical Michaelis-Menten velocity equation: Vo = (Vmax/Km +c(S) ) becomes:
Vo = (Vmax/Km + c(S)) = Michaelis-Menten,
uncompetitive inhibitor equation
The constant b is equal to 1 in
uncompetitive inhibition, and thus it is essentially absent from the equation. The constant c has a non-zero value that is greater than 1, the constant depends on the inhibitor.
As you can see this means that at very high substrate concentrations Vo will approach Vmax/c; thus Vmax is decreased. Apparent Km will also decrease because, as we know, Km is (for practical purposes) the substrate concentration required to reach 1/2Vmax. In the equation above laid out for uncompetitive inhibition, we see that for Vo = Vmax/2 we must have 2 = Km + c(S). Thus the substrate concentration required to reach 1/2Vmax decreases by a factor of c, and the "apparent Km" decreases.
A mixed inhibitor is a separate entity from both the
uncompetitive inhibitor and the
competitive inhibitor.
A
mixed inhibitor also binds at a site on the enzyme which is physically separate from the active site, however it can bind both to the enzyme itself and to the enzyme-substrate complex. As a brief aside the nomenclature for an enzyme-substrate complex is (ES), for the enzyme-inhibitor complex it is (EI) and for the enzyme-substrate-inhibitor complex it is (ESI).
Like uncompetitive inhibitors, a
mixed inhibitor will usually affect both Km and Vmax.
For a mixed inhibitor the equation Vo = Vmax/bKm + c(S) holds true.
In this equation, for a
mixed inhibitor, the constants b and c both have a non-zero value.
In the case where b = c, the particular mixed inhibitor is called a
noncompetitive inhibitor.
Noncompetitive inhibitors will decrease Vmax but leave Km unaffected. As substrate concentration increases such that it is much greater than Km, then Vo will approach Vmax/c, the same case we saw when discussing the uncompetitive inhibitor. However, because for a noncompetitive inhibitor b = c, Km will not change.
Again, the numerical value of Km is the substrate concentration required to reach 1/2Vmax. So for Vo=Vmax/2 we must have 2 = bKm + c(S). Because b and c are the same, both Km and (S) are multiplied by the same factor, and the apparent substrate concentration required to reach 1/2Vmax does not change.
In summary:
Michaelis-Menten velocity equation: Vo = Vmax/bKm + c(S)
Competitive Inhibitor: b = non-zero, greater than 1; c = 1
Apparent Km = increased; Vmax = decreased
Uncompetitive Inhibitor: b = 1, c = non-zero, greater than 1.
Km = decreased; Vmax = decreased
Mixed inhibitor: b = non-zero, greater than 1
c = non-zero, greater than 1
Vmax = decreased; Km = decreased
Noncompetitive Inhibitor
b = c = non-zero, greater than 1
Vmax = decreased, Km = unaffected
And for the record I learned all of this from reading Lehninger Principles of Biochemistry and would encourage you to read that book for any/all biochemistry concerns you might have. It literally has anything and everything you could ever want to know about biochemistry as it pertains to the MCAT, and much much more.
