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PencilScience JournalAgendasMacromolecules Worksheet (Pass it forward if you have not done so already).Plan for the week

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Science Question of the Day

Explain why the chemical bonds in the body do not break down, spontaneously, releasing all of their energy.


5.3 Chemical Reactions

Chemical reactions begin with reactants (substrates) which are converted to products

Exergonic reactionsProducts contain less energy than the reactants

Endergonic reactionsProducts contain more energy than the reactants


Energy of ActivationExtra energy required to destabilize chemical bonds and so initiate a chemical reaction

Fig. 5.4


Catalysts lower the activation energy of a reaction, and thus increase its rate

However, they cannot make an endergonic reaction exergonic

Fig. 5.4


5.4 How Enzymes Work

Enzymes are proteins that serve as catalystsThey speed up chemical reactions within cells

Enzymes bind a specific molecule and stress bonds to make a particular reaction more likely

Active siteSite on enzyme surface where reactant fits

Binding siteSite on reactant where enzyme binds


Fig. 5.5 Enzyme shapes determines its activity

Lysozyme

Changes upon binding of the

substrateThe substrate is now bound more

intimately


Fig. 5.6 How enzymes work


Fig. 5.7 The catalytic cycle of an enzyme

1 The substrate, sucrose, consistsof glucose and fructose bonded together.

Bond

Enzyme

Active site

The substrate binds to the enzyme, forming an enzyme-substrate complex.

2

H2O

The binding of the substrate and enzyme places stress on the glucose-fructose bond, and the bond breaks.

3

Glucose Fructose

Products are released, and the enzyme is free to bind other substrates.

4


5.5 Factors Affecting Enzyme Activity

Enzyme activity is affected by any change in condition that alters the enzyme’s 3-D shape

The structural bonds of enzymes are sensitive to changes in temperature and pH

Therefore, a temperature or pH beyond the optimal range will cause the enzyme to denature


Enzyme Activity

http://www.kscience.co.uk/animations/model.swf


Fig. 5.8 Enzymes are sensitive to their environment

Digests proteins in

the intestine

Digests proteins in

the stomach


5.6 How Cells Regulate Enzymes

A cell can control the activity of an enzyme by altering its shape

Allosteric enzymes have shapes that can be altered by the binding of signal molecules

These molecules bind to the allosteric site

Repressors bind and repress enzyme activity

Activators bind and restore or increase enzyme activity


Fig. 5.9 How cells control enzymes


Enzyme inhibition occurs in two ways:Competitive inhibition- Inhibitor binds at the

enzyme’s active site

Noncompetitive inhibition- Inhibitor binds at the

enzyme’s allosteric site

Fig. 5.10

http://www.elmhurst.edu/~chm/vchembook/573inhibit.html

http://www.elmhurst.edu/~chm/vchembook/573inhibit.html


Enzymes Reviewed

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