B

B. Enzymes - Biological Catalysts

1. Activation Energy & the Role of Enzymes

Activation Energy = amt of energy put into a rxn in order to break bonds

Ex. Nitrogen fixation

N₂ + H₂ à NH₃ G= - 19 kcal/mole

In Industry - 450° C, 400 atm

Nitrogenase - ambient temperatures, 1 atm

2. Characteristics of Enzymes

protein catalysts - incr activation energy of rxn
speed up rxns by up to 10¹⁰ times
contain highly 3-dimensional stereospecific active sites

3. Enzyme Nomenclature

1.Named for its substrate: substrate+ase

Ex. lactase

Catalyzes lactose à glucose + galactose

2.Named for its action

Ex. Deoxyribonuclease, or DNase catalyzes DNA à dNMP nucleotides

4. Enzyme Structure

proteins, polymers of aa
range in size from 1 x 10⁴ daltons to 1 x 10⁶ daltons w/ most being in the 10⁵ range
may be simple - protein alone
or conjugated - protein + nonprotein
protein portion of an enz = apoenzyme
non-protein part of an enz = cofactor

cofactors can be loosely bound coenzymes

Exs. NAD, Coenzyme A, many vitamins

or tightly bound prosthetic groups

Exs: folic acid, riboflavin, FAD, heme groups, Zn, Mb, etc.

complete enz (apoenzyme + cofactor) = holoenzyme

5. Enzyme Function

convert a target molecule, the enzyme's substrate(s), into a different molecule, the enzyme's product(s)

efficient & specific

enz & substrate fit into each other like a lock into a key

enz increase the rate of rxn by lowering the activation energy

they lower this activation energy by binding the substrates of the rxn

this causes a rxn to occur for 2 reasons:

1.Enz bind their substrates at what is called the active site. When bound at the active site the substrates are brought into close proximity to one another & will react

2.The enz will bend the substrates in such a manner to make them more reactive. This makes it possible for substrates to react and form products.

6. Enzyme Location

enz are organized by the cell into collections that carry out multi-step conversions
each enz in the collection will take its substrate, convert it to product & hand it off to the next enz
these collections are called biochemical pathways

Key Point - enzymes catalyze (speed up) the rxns of the cell