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Concepts in Biochemistry

Third Edition

Chapter 6:

Enzymes II: Cofactors, Regulation, andCatalytic RNA

Copyright 2006 by John Wiley & Sons, Inc.

Rodney Boyer

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Cholineessential

nutrient

Rapid brain

development, enhance

memory and slows

progression of

Alzheimers disease;improves liver and

cardiovascular

function

Rich sources: fish,

eggs, soybeans, liver,cauliflower, peanuts

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Enzyme and coenzymes

Some enzymesbiologically active in their

native state

Some require cofactors for activity

Holoenzyme = apoenzyme (protein) +

cofactor

Cofactors: metal ions; small organic molecule

called coenzymesvitamin derivatives

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Enzyme regulation

Enzyme activities in metabolic pathwaysare subject to regulation

Allosteric enzymesregulatory enzymes;- Influenced by noncovalent binding of signal

molecules (positive effectors or negative

effectors)- Oligomeric (several subunits)

- Have both catalytic and regulatory sites

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Homotropic allosterism

Substrate and effector are the same type of

molecule

Several catalytic sites; identical subunits

The binding of one substrate to a catalytic

site influences the binding of other

substrates to the catalytic site (positivecooperativity)

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Sigmoidal

curve

characteristic

of positivecooperativity

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Heterotropic allosterism

Effector molecule is different from the

substrate

regulatory subunit and catalytic subunit aredifferent

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MWC concerted model of positive

cooperativity

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Sequential model ofpositive

cooperativity by

Koshland

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Other forms of enzyme regulation

Covalent modification of enzymes

chemical alteration of the enzyme

- phosphorylation/dephosphorylation of OHgrps of serine, threonine or tyrosine

- adenylylation

- Reduction of disulfide bonds

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Activation by proteolytic cleavage of a

proenzyme

Zymogensinactive precursor of enzymes

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Regulation by different molecular forms -

isoenzymes

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