Chapter 6.4

Enzymes Don’t Work In A Vacuum

AP BiologyFall 2010

Bell Ringer

Objectives

• Describe allosteric site and feedback inhibition

• Understand the effects of temperature, pH, and salinity on enzymes

• Know the ways cofactors help enzymes function

Controls Over Enzymes

• Cells produce what conditions require- no more, no less

• Allosteric site: a specific receptor site on an enzyme molecule remote from the active site, molecules bind to the allosteric site and change the shape of the active site making it less receptive to the substrate

Controls Over Enzymes

• Feedback inhibition: a cellular control mechanism in which an enzyme that catalyzes the production of a particular substance in the cell is inhibited when that substance has accumulated to a certain level, thereby balancing the amount provided with the amount needed

Controls Over Enzymes

• Ex. Feedback loop starts and ends at many allosteric enzymes

• Unused tryptophan binds to an allosteric site on the first enzyme in the tryptophan biosynthesis pathway

• Binding makes the active site change shape, so less tryptophan can be made

• When less tryptophan around, allosteric sites unbound– Active sites remain functional, and synthesis rate picks up

• Feedback loops adjust to concentrations of substrates

Effects of Temperature, pH, and Salinity

• Enzymes operate best within defined temperature ranges

• High temperatures decrease reaction rate by disrupting bonds that maintain 3-D shape – Denaturation

Effects of Temperature, pH, and Salinity

• Most enzymes function best at a pH near seven

• Higher or lower values of an enzymes optimal pH value disrupt enzyme shape and halt function

Effects of Temperature, pH, and Salinity

• Range of salinity tolerance for most enzymes• If enzymes are dissolved in fluids exceeding

that tolerance, the hydrogen bonds are disrupted, inactivating the enzyme

Help From Cofactors

• Cofactors: metal ions or coenzymes that bind to many enzymes and make them more reactive – Organic compounds that may or may not have a

vitamin components• Metal ions readily give up and accept

electrons, and are able to help products form by shifting electron arrangements in substrates

Help From Cofactors

• Antioxidants like vitamin E and catalase work to help neutralize free radicals – Highly reactive atoms with unpaired electrons

• Inorganic metal ions such as Fe++ also serve as cofactors when assisting membrane cytochrome proteins in the electron transfers in chloroplasts and mitochondria

Review

1. Describe how an allosteric site prevents enzyme function.

2. Describe feedback inhibition.3. Describe 3 properties that effect enzyme

functionality.

Answers

1. Allosteric site: a specific receptor site on an enzyme molecule remote from the active site, molecules bind to the allosteric site and change the shape of the active site making it less receptive to the substrate

2. Feedback Inhibition: a cellular control mechanism in which an enzyme that catalyzes the production of a particular substance in the cell is inhibited when that substance has accumulated to a certain level, thereby balancing the amount provided with the amount needed

3. Temperature, pH, salinity