Enzymes

Chemical reactions depend on molecules being properly oriented and of sufficient energy to cause chemical changes to occur. (Demonstration)

The rate of reaction can be accelerated through the use of catalyst. (Demonstration).  Catalyst operate by lowering the energy needed to activate the reactive molecules (Fig. 1 or Figure 2.22 in Text).

Biological reactions are accomplished by catalyst called enzymes - proteins that possess specific catalytic activities.

The term enzyme actually means "in yeast".

There are more than 2000 different known enzymes.  Nearly every chemical reaction is catalyzed by an enzyme.
 
Molecules used in the fermentation of sugar to form alcohol were the first enzymes to be isolated.

Enzymes catalyze the series of reactions by which metabolic pathways breakdown compounds obtained from the environment; and they degrade and reconstruct the components needed to maintain the organism.

An enzyme may create an environment in which the equilibrium of a particular reaction is reached more rapidly than is possible by spontaneous reaction, but it cannot alter the equilibrium itself.

 Certain catalysts known as ribozymes are made of RNA. At this point in the course, however, we are only dealing with enzymes which are proteins.
 

A typical enzyme accelerates a reaction 10^8 to 10^10 fold. Some can increase as high as 10^15 fold.

In order for a chemical reaction to proceed, the substrate must first be converted to a higher energy state, the transition state. The energy required to reach the transition state is the activation energy.

Enzymes are highly specific, each catalyzes only a single reaction or set of closely related reactions.

Ex: Beta-galactosidase is moderately specific. It will split lactose but it will also split any disaccharide joined with a beta-galactoside bond.

 Phosphorylase kinase acts with absolute specificity with a single substrate - glycogen phosphorylase.

Enzymes are not altered in the net chemical reaction.  Occasionally covalent bonds involved, but usually enzymes use noncovalent interactions.  Even when covalent bonds are used, the enzyme will revert to its original state at the end of the reaction (Fig. 2.26 in text).

 In any enzyme catalyzed reaction, the reactants always forms a tight complex (have an affinity) with the enzyme:

substrate S
enzyme E
product P
enzyme * substrate complex forms = ES complex
enzyme * product complex forms = EP complex

 E + S <----------> ES <------------> E P <-----------> E+ P

 Reactions are reversible.

 

 ACTIVE SITE - site on the enzyme where the substrate binds. Often the active site is an actual cleft or groove in the 3D structure of the enzyme (Fig. 2.25 in text).

 Linear array of amino acids which make up active site are dispersed throughout the primary structure. Secondary and tertiary folding allow the amino acids which make up the active site to come together.

 After the ES complex forms, the substrate is usually altered in some way that facilitates further reactions.

 1 or more transformations occur leading to product formation and dissociation of the product from the enzyme.

 Affinity - refers to the strength of the binding interaction at the active site.

 Specific amino acids in the active site (especially acidic and basic amino acids) may form temporary bonds with reaction intermediates.

 In addition, the active site may bind other small molecules that participate in the catalysis.

--prosthetic groups - small molecules bound to proteins (zinc, iron common) Insulin 3D view and Quiz

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