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, converts sucrose into glucose and fructose.
Food Uses include:

Production of artificial honey
Production of invert sugar
Manufacture of liquid center candies

Lactase

Beta-Galactosidase
This enzyme converts lactose into glucose and fructose. It has the following applications:
Prevention of sandiness in ice cream by prevention of crystallization of lactose

Reduction of lactose intolerance
Improving the freeze stability of milk proteins
Decreased time for cheese manufacture
Improved efficiency of biological utilization of lactose

Glucose Oxidase-Catalase


Removal of oxygen ( or glucose ) from products to prevent oxidation or browning including:

Beer
Cheese
Dried eggs
Fruit Juices
Meat and fish
Milk powder
Wine

In the above reaction, glucose was converted to Gluconic acid. In the process oxygen was converted into hydrogen peroxide.

Polyphenol Oxidase

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.

Pectinases

Types of pectic enzymes;

Pectin methylesterases
Polymethylgalacturonases
Pectic lyases

Uses of pectic enzymes:

Hydrolysis of pectic substances
Assist in the fermentation of cocoa
Hydrolysis of coating of coffee beans to assist fermentation
Softening of fruits
Extraction of oil from olives
Improved yield of fruit juices
Prevent cloudiness in fruit juices
Clarification of wine