Enzymes and Their Requlation

Biological catalysts--active sites




Types of Enzymes

            Exoenzymes--affect extracellular nutrients

            Endoenzymes--affect absorbed nutrients to provide energy

Chemical and Physical Properties of Enzymes

            Protein--Protein / other chemical group

                        denatured by heat

                        precipitated by EtOH or  inorganic salts


                        MW 10,000 to 1,000,000

            Holoenzyme--coenzyme and apoenzyme

                        apoenzyme--protein portion of coenzyme

                        coenzyme--low MW organic molecule and protein

                        cofactor--non-protein portion of enzyme

                                    Mg, Mn, Fe, Zn, other metals

            Each enzyme causes a one-step change in the substrate

                        enzyme systems--multienzymatic pathways

            Enzyme Nomenclature and Classification

                        Comm on Enzymes of the Inter Union of Biochemistry

                                    -ase suffix (single enzymes)

                                    type of chemical Rx catalyzed is basis for class.

                                    trivial and systematic names

                                                substrate--reaction catalyzed—hexoskinase

                                                            ATP: hexose phosphotransferase

            Nature and Mechanism of Enzyme Action

                        Substrate activation

                                    high chemical affinity

                                    strain / distortion (labile) linkage in substrate

                        Endoenzymes have more than one active site

                                    lactate dehydrogenase--4

                        Exoenzymes have one active site

                        Activation energy--lowered by enzyme


            Conditions Affecting Enzyme Activity

1)      Concentration of Enzyme

2)      Concentration of Substrate

3)      pH

4)      Temperature




Inhibition of Enzyme Action

            Non-reversible--mod. / inact. of functional group of enzyme


                        Competitive--reversed by incr. substrate conc.

Non-competitive--“ties up” cofactors / coenzymes

Conditions Affecting Enzyme Formation

            Organisms change Rx to environment within limits

                        Constitutive--always produced by cells

Induced--substrate induces production


Natures and Mechanisms of Enzyme Regulation

1)      Direct Control

Control of Catalytic Mechanisms

            substrate / reactant conc.--limiting factors

            compartmentalization--member bound enzymes

            proteolytic enzymes--degrade other enzymes

Control through Coupling with other processes-ligands

            feedback inhibition--end product of pathway

            precursor activation--activates last E of path

            energy link control--high energy charge inhib.

            regulatory enzymes--allosterism

2)      Genetic Control

Repression--low MW corepressor / repressor complex

            Corepressor combines with repressor protein

                        Operator gene blocked by complex

Induction--low MW substance “ties up” repressor

i gene--repressor gene:  creates repressor protein

p gene--promoter gene:  cAMP activates CAP

            catabolite gene activator protein

            activates transcription of the operon

o gene--operator gene:  activates structural genes

Negative control--Reprossor / corepressor block o-gene

            blocks transcription (induction or repression)

Positive control--cAMP and CAP operating on p-gene

            glucose reduces cAMP in lac operon