Enzymes possess extraordinary specificity and catalytic power.
> 2000 different enzymes known.
Term actually means "in yeast"
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.
Enzymes then are special proteins which are able to catalyze chemical reactions. Biological catalysts.
1) A typical enzyme accelerates a reaction 10^8 to 10^10 fold. Some can increase as high as 10^15 fold.
2) Enzymes accelerate chemical reactions by lowering the activation energy required.
See Figure 2.22 in Text
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.
3) 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.
4) Nearly every chemical reaction is catalyzed by an enzyme.
5) Enzymes are not altered in the chemical reaction.
In any enzyme catalyzed reaction, one of the reactants always forms a tight complex with the enzyme.
substrate S
enzyme E
The E and S have an affinity for one another
enzyme * substrate complex forms = ES complex
E + S <----------> ES <------------> E + P
Reactions reversible.
Occasionally covalent bonds involved, but usually noncovalent interactions.
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.
*Linear array of amino acids which make up active site are dispersed. Secondary and tertiary folding interactions which we covered in the previous lecture, 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
Two hypotheses for the way enzyme-substrate interact:
*lock and key (complementarity)
*induced fit (active site becomes complementary only after the initial binding of enzyme to substrate).
It turns out to be one or the other hypotheses works for every enzyme system known. Induced fit seems to be most common. The substrate is placed under strain when enzyme binds, making the substrate more likely toead to further reactions.
Enzymes can bring 2 or more substrates together, distort their conformation- allowing them to interact.
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)
Coenzymes - small molecules which often work together with enzymes to enhance the reaction rate. Coenzymes tend to be low mw organic molecules which serve as carriers for several types of chemical groups.
Functions as a carrier of electrons in oxidation- reduction reactions. NAD may accept a [H+] and 2 electrons to become NADH from 1 substrate and then donate the electrons to a second substrate.
Other examples of coenzymes which are electron carriers: NADP, FAD, etc.
Other coenzymes may be involved with the transfer of carboxyl and acyl groups.
Some coenzymes may function with a variety of different enzymes.
Many coenzymes are closely related to vitamins. The vitamin often contributes all or part of the structure of certain coenzymes. Although not required for prokaryotic cells, vitamins are required in the diets of humans and other higher animals.
Activity can and really must be regulated.
Feedback Inhibition
very simple
When the product of a metabolic pathway inhibits the activity of an enzyme involved in its synthesis.
See Example of Feedback inhibition given in text
Allosteric Regulation
The regulatory molecules bind not to the catalytic site (active site) but to a different site on the enzyme. Feedback inhibition is one example of this form of regulation.
allo = other
steric= site
Binding of the regulatory molecule changes the conformation of the protein, which in turn alters the shape of the active site and the catalytic activity of the enzyme. The regulatory molecule may serve as an activator of enzyme activity rather than an inhibitor.
Enzymes can also be regulated by covalent modifications such as Phosphorylation. Phosphorylation may stimulate or inhibit enzyme activity, depending on the individual enzyme.
The phosphate for phosphorylation is supplied by ATP.