Properties of Enzymes

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Properties of Enzymes
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Enzymes are catalystsWhat properties wouldideal
catalysts have?
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Enzymes are catalystsWhat properties wouldideal
catalysts have?

1. High degree of specificity for their substrates.
2. Accelerate chemical reactions tremendously.
3. Function in mild conditions.
4. Be recycled to participate again.

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A Few Definitions
Cofactor additional chemical component needed
for catalysis. - often an inorganic metal ion
(mineral). Coenzyme complex organic molecule
needed for catalysis. - often a
vitamin. Prosthetic group non amino acid
portion of the enzyme needed for catalysis.
Often a coenzyme or metal ion. Holoenzyme
complete catalytically active enzyme, with
all necessary prosthetic groups. Apoenzyme The
protein part of the holoenzyme. Prosthetic
groups are absent.
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Six Classes of Enzymes

1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases

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Stored electrons
One bond to oxygen
Two bonds to oxygen
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Amino group transferred
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Water did chemistry to break a bond
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CO2 was removed
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Amino group switched places
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Two substrates were ligated together
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Enzyme Kinetics
E enzyme S substrate P product ES
enzyme-substrate complex k rate constant
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The rate of reaction is dependent on enzyme
concentration Enzyme ltltlt substrate Figure 5.2
Velocity, or how fast the reaction is going
Concentration of enzyme
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When measuring ratesof enzyme-catalyzedreactions
, initialvelocity (vo) is measured. Figure 5.3
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Enzyme kinetics terminology
S substrate concentration Vo initial
velocity of a reaction. A significant amount of
substrate has not yet been converted to
product. Vmax maximal velocity of a reaction.
Addition of more substrate will not increase the
rate of the reaction. Km The concentration of
substrate at which the rate of the reaction is
half-maximal
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Michaelis-Menten equation
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The rate of reaction is dependent on substrate
concentration Figure 5.4
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Km values are often just above the substrate
concentrationin a cell. Rates of reaction are
sensitive to small changesin cellular substrate
concentrations.
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kcat is a measure of the number of substrate
moleculesconverted to product per second per
enzyme molecule
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Experimental determination of kinetic
constants Figure 5.6
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Reversible Enzyme Inhibition
An inhibitor is a compound that binds to an
enzyme andinterferes with its activity. Many
drugs are enzyme inhibitors.
An inhibitor is characterized by an inhibition
constant (Ki).
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No Inhibitor
Figures 5.8 and 5.9
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No Inhibitor
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Many competitive inhibitorsare substrate analogs
Benzamidine is an inhibitor of the enzyme trypsin
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Many competitive inhibitors are substrate
analogs. Compound (b) designed as an inhibitor of
the enzymepurine nucleoside phosphorylase, that
utilizes guanosine (a) asa substrate. (b) is a
possible drug for the treatment of
arthritis. Figure 5.13
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Irreversible Enzyme Inhibition
Some inhibitors are compounds that form a stable
covalentbond with the target enzyme.
DFP inactivates serine proteasesby covalently
modifying anactive site serine residue. Figure
5.15
F-
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Regulation of enzyme activity by metabolite
concentration
Activator
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Regulator ADP binds at an allosteric
site, Separate from the active site
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The activity of the some enzymes isregulated by
reversible phosphorylation. Example pyruvate
dehydrogenase
Figure 5.22