Types of enzymes

Exoenzymes

Extra-cellular action

Hydrolysis

Penicillases, cellulase, amylase

 

Endoenzymes

Intra-cellular action

Most are of this type (Fig. 8.7)

 

Constitutive

Always present and in constant amount in cell regardless the amount of substrate (Fig. 8.8a)

 

Regulated enzymes

Not in constant amounts in cell

Produced only when substrate is present (inducible)

Turned off when substrate is absent (repressed) -- Fig. 8.12

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Factors affecting enzyme activity

Temperature

The higher the more unstable or labile

pH and chemicals

Osmotic pressure

Heavy metals

Denaturation

Occurs when weak bonds of apoenzyme are broken

Distorts the shape of the enzyme

Prevents the substrate from attaching to the active site

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Transfer reactions

Add or remove functional groups

A molecule can be:

Oxidized

Loses or gives or donates electrons

Liberation of energy

Reduced

Gains or receives or accepts electrons

Gains energy

 

"Redox" reactions are common in microbial cells and indispensable for life processes

Oxireductases

Group of enzymes that can remove electrons from one substrate and add them to another

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Tpes of enzymes according to the chemical group transfer they perform

Aminotransferases
NH₃⁺

Phosphotransferases
PO₄^3-

Methyltransferases
CH₃

Decarboxylases
CO₂

Dehydrogenases
H⁺

Transferases
C, N, P, S

Hydrolases
H₂O

Isomerases
Isomeric

Lyases
Double bonds

Ligases
Form bonds

Lipases
Fats

Deoxyribonucleases
DNAase

Synthetases/polymerases
Synthesis

Cellulase
Cellulose

Lactase
Lactose

Aldolases
Aldehydes

Oxidases
Oxidation

 

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Key common words that describe the action of enzymes:

Add, remove, cleave, bond, hydrolize, dehydrolyze, change, alter, break, catalyze, digest, transform, attach, synthesize, convert, etc.

Some important concepts:

Metabolic reactions proceed in a systematic, highly regulated manner that maximizes the use of available nutrients and energy

Regulation of metabolism is the regulation of enzymes by an elaborate method of checks and balances

Metabolic reactions occur in a multi-step series or pathway, with each step catalyzed by an enzyme

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Patterns of pathways

Linear

Cyclic

Branched

Direct control on the behavior of enzymes

Competitive inhibition

Other molecules with a structure similar to the normal substrate can occupy the enzyme's active site

Feedback control

End product being fed back into the system negates (cancels) an enzyme's activity

A + B ------- Enzyme ------> C

C will build up and cancel the catalytic action of the enzyme

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Allosteric enzymes

Have an additional regulatory site for the attachment of molecules other than the substrate. Distort active site so no binding to substrate occurs without denaturation

Control of enzyme synthesis

Enzyme repression

Excess product turns off genetic program in DNA

Enzyme induction

Enzyme appear only when suitable substrates are present

Synthesis induced by its substrate

Adaptation to the environment, e.g.:

E. coli will produce lactase in the presence of lactose to yield glucose and galactose

E. coli will produce sucrase in the presence of sucrose to yield glucose and fructose

 

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Bioenergetics

Energy

Capacity to do work or cause change

Potential (not spent)

Kinetic (freed)

Cells extract energy from bonds and apply it towards useful work

If energy is released then reaction is exergonic

If energy is absorbed then reaction is endergonic

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Adenosine triphosphate (ATP)

High energy phosphate molecule

Stores and releases energy

Unique molecular structure

A temporary energy repository

Bond releases energy when broken

Negatively charged (PO₄^3-)

Other properties of ATP

The high energy charge originates in the orientation of the phosphate groups

Negative charges impose strain on bonds

Removal of the terminal PO₄^3- releases the bond energy

Formed by substrate level phosphorylation and photophosphorylation

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Textbook: Foundations in Microbiology. K.Park Talaro. 6th edition. McGraw Hill.

Remember to read your textbook, study tables, graphs and illustrations.
Develop a strategy to administer your time so that when exams come you do not have to cram.
Attend lectures and ask questions.

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