ENZYMES SBI4U0. ENZYMES An enzyme is a biological catalyst –Biological = protein –Catalyst = speeds up a reaction without being consumed or permanently.

ENZYMES

SBI4U0

ENZYMES• An enzyme is a biological

catalyst–Biological = protein–Catalyst = speeds up a reaction without

being consumed or permanently altered in the process

• All enzymes are proteins, but not all proteins are enzymes

• All enzymes are catalysts, but not all catalysts are proteins

• Examples?

ACTIVATION ENERGY

• Every chemical reaction involves both breaking and forming chemical bonds– Breaking bonds requires energy– Forming new bonds releases energy– (Sometimes net release, sometimes net requirement)

• Even though a particular reaction may be spontaneous, a small amount of energy must be initially invested– “Activation energy”

• How do you earn interest at your bank?

• Free energy is required to break bonds–Activation

energy• Free energy is

released when new bonds are formed

ENZYMES

• Enzymes speed up chemical reactions by lowering their activation energy– The net change in free energy remains

unchanged

SUBSTRATE

• The reactants in an enzyme-catalyzed reaction are termed “substrates”

Substrate product

Sucrose glucose & fructose

Starch glucose

Deoxynucleotide triphosphates DNA

Etc.

ENZYME SPECIFICITY

• No single type of enzyme can catalyze all chemical reactions

• Enzymes display specificity as to what chemical reaction they catalyze– “Substrate specificity”

• Specific enzymes catalyze each of the following reactions– Sucrose glucose & fructose – Starch sugars– Deoxynucleotide triphosphates DNA

ENZYME STRUCTURE

• Enzymes are proteins with specific three-dimensional shapes– Defined by chemical bonds– 1o, 2o, 3o, & 4o structures

• The portion of an enzyme that binds to the substrate is the “active site”– Complementarity of fit– Lock-and-key fit– Induced fit (shaking hands)

REACTION RATE• The rate of an enzyme-catalyzed

reaction is dependent upon several factors–[Substrate]–[Enzyme]–Temperature–pH–Cofactors–Inhibitors

[SUBSTRATE]

• The rate at which an enzyme converts substrate into product can be increased by increasing substrate concentration– There is a limit

• At some substrate concentration all of the enzymes’ active sites are bound to substrate– The enzyme is “saturated”– Adding more substrate will not increase the rate– Adding more enzyme will increase the rate

TEMPERATURE & pH

• An enzyme’s three-dimensional shape is defined by various chemical bonds–Altering the temperature

or pH can interfere with these bonds

–Altered enzyme shape compromises activity

– “Denaturation”

COFACTORS

• Many enzymes require non-protein helpers for catalytic activity– “Cofactors”

• Cofactors may be organic or inorganic– Inorganic cofactors are typically metal ions– Organic cofactors (“coenzymes”) are typically vitamins

or are derived from vitamins

• e.g., DNAse requires Mg2+ as a cofactor– Removal of Mg2+ inactivates the enzyme

INHIBITORS

• Certain chemicals can selectively inhibit the activity of specific enzymes– May be reversible or irreversible– Generally weak vs. covalent bonds

• Competitive inhibitors– Compete for binding to active site– Increased [substrate] can overcome

• Noncompetitive inhibitors– Binding elsewhere alters shape,

alters active site

Inhibitors:

A substrate competitive Noncompetitive normally to inhibitor inhibitor an active site

REGULATORS

http://www.nbotr12.nelson.com/Biology12/Attachments/d_Animations/allosteric.mov

Allosteric enzymes: 2 different binding sites

One for substrate (active site) and one for allosteric regulator (allosteric site)

Regulator binding changes substrate active site shape. Allosteric regulation can activate or inhibit.

INHIBITORS

• Many poisons act by inhibiting enzymes– e.g., DDT inhibits key enzymes in the nervous

system– e.g., many antibiotics inhibit (penicillin, etc.)

key bacterial enzymes

• Selective inhibition of key enzymes is an essential mechanism of metabolic control

FEEDBACK INHIBITION

• Anabolic biosynthetic pathways produce an end-product such as an amino acid

• This end-product should be produced only when needed

• In many cases, this end-product acts as an inhibitor of an enzyme functioning early in the biosynthetic pathway– “Feedback inhibition”

http://www.nbotr12.nelson.com/Biology12/Attachments/d_Animations/feedback.mov

Complete review questions (section 1.4) #1-8 page 77

(b) Capsaicin’s intense flavour results from the molecule’s long hydrocarbon tail. The chain allows it to bind very strongly with its lipoprotein receptor, which has some hydrocarbon side chains of its own (like dissolves like!). The fatty tail also allows the molecule to slip through lipid-rich cell membranes, making the burn more persistent.

• c) The perception that peppers are “hot” is not an accident. Capsaicin allows calcium ions to move through the cell membrane from the extracellular fluid into the cell. That ultimately triggers a pain signal that is transmitted to the next cell. When the cells are exposed to heat, the same events occur. Chili burns and heat burns are similar at the molecular, cellular, and sensory levels.

• (d) Capsaicin is found primarily in the pepper’s placenta—the white ribs that run down the middle and along the sides of a pepper. Since the seeds are in such close contact with the ribs, they are also often hot.

• (e) Capsaicin content is measured in parts per million and are converted into Scoville heat units, the industry standard for measuring a pepper’s intensity. One part per million is equivalent to 15 Scoville units. Bell peppers have a value of zero Scoville units; habaneras—the hottest peppers—register 200 000 to 300 000 Scoville units. Pure capsaicin has a Scoville heat unit score of about 16 million.

• (f) Capsaicin, which is a largely nonpolar molecule, does not mix with water. Drinking water will have little effect on the burning sensation. The most useful liquid is milk (rinsing the mouth with it as you sip) or eating rice or bread, which absorb the capsaicin.

(g) As a medicine, capsaicin is used to help relieve the pain associated with neuralgia. Capsaicin is also used to temporarily help relieve pain from arthritis. It is also used as a repellent against dogs, birds, bears, and squirrels, and as an insect repellent. It is the active ingredient in commercially available “pepper sprays.”