Enzymatic Reactions
At equilibrium ( steady state, where concentrations are not changing) can define the rate of enzyme-substrate complex formation by using the equilibrium constants:
K1[E] [S] + K4 [E] [P] = [ES] ( K2+ K3 )
At time zero, [S] >>>[P]
then :
K1[E] [S] = [ES] ( K2+ K3 )
Can rearrange to:
E/ES = K2 + K3/ K1
Km = K2 = K3/ K1
Km = Michaelis Constant
With proper substitutions and rearrangement of the rate equation we can obtain the Michaelis-Menton equation:
It can be shown that:
V = 1/2 Vmax , when Km = [S]
When the substrate concentration is equal to Km, the reaction will proceed at half maximal velocity. If we take the reciprocal of the equation and plot 1/V versus 1/S we obtain a Lineweaver-Burk Plot:
Factors that affect enzyme activity:
pH
Temperature
Ionic
Strength Aw
Substrate Concentration
Substrate location.
Enzymes usually have a fairly narrow pH optimum, but they often
show activity 2 or 3 units away from that optimum. pH may alter:
Enzyme conformation
Recognition site
Active site
Substrate conformation
The effects of temperature on enzyme activity may be multiple and
may include:
Reaction rates and energy of activation
pH effects
Denaturation effects
We have discussed reaction rates. Can determine Ea for enzymatic
reactions. The greater EA, the more temperature the reaction is.
pH
As temperature changes the ionization of charged groups also
changes and this results in changes in pH. These may be desirable
or undesirable. Temperature - pH relationships are often
neglected in the study of enzymes.
Food enzymes are:
Associated with loss of product quality
Used to evaluate process induced changes
Used to enhance flavor quality
Used to alter physical properties
Used to modify protein functionality
Enzymes are used because they are:
Selective for both substrate utilized and product formed
Effective under mild conditions
Easy to control
Amylases
Some food uses of amylases include:
Syrup manufacture
Dextrose manufacture
Baking
Saccharification of fermented mashes
Food dextrin and sugar product manufacture
Dry breakfast food manufacture
Chocolate syrups
Starch removal from fruit juices
Alpha amylases cleave internally - little change in sweetness, large decrease in viscosity. Beta amylases cleave maltose units from the ends - large change in sweetness, little decrease in viscosity. Glucoamylases cleave glucose units from the ends- large change in sweetness, little decrease in viscosity
Glucose Isomerase
The enzyme, glucose isomerase converts glucose to fructose as shown below:
Invertase
The enzyme, invertase<!--invertase-->, converts sucrose into glucose and fructose.
Food Uses include:
The enzyme polyphenol oxidase is
responsible for a number of off colors that develop in
fruits and vegetables. The enzyme can add a hydroxyl
group to phenolic compounds or oxidize polyphenolics to
the corresponding ketones.
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