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Enzymes
Demo – scissors used to cut paper, stapler used to put together, both not changed
Metabolism – (Greek for change) all chemical processes
Enzymes are globular protein catalysts. Catalysts increase the rate of a chemical reaction
without being consumed or used up themselves.
Reactions require bonds that are either broken or reformed. The energy needed to break
the bonds is called the activation energy. (EA)
Ea is a good thing as most complex molecules in the body, including DNA and
proteins are rich in energy. If no barrier, they would spontaneously decompose.
Enzymes control this.
High temperatures can supply the energy but we couldn’t survive the temperature.
Enzymes speed up a reaction by lowering the EA without the high temperatures. They do
this by forming a transitional state with a slightly different configuration that decreases
the EA. (puts the substrate in the right place and weakens the bonds a bit so things can go
faster)
Substrate – the molecule the enzyme catalyses
Active site – the actual part of the enzyme that binds to the substrate. Together they form
the enzyme-substrate complex. (Being a globular protein allows for lots of different
places to form an active site. All the folding allows for more surface area.)
Enzymes have substrate specificity – meaning…..
How do enzymes achieve this substrate specificity? Lock and Key model. Explains why
cells have glycoproteins and peripheral proteins to allow for this.
The body uses enzyme inhibition a lot to control enzymes. You don’t want them
catalyzing everything they see. You want to control them and regulate your metabolic
activities. DO you want all your food broken down into glucose right now and shipped to
the muscles, or all your amino acids made into proteins right now!
Factors that affect the rate of enzyme – controlled reactions
• temperature – generally, as temperature increases, so does the reaction rate – every 10°
C rise, the rate doubles. Because molecules move faster and can find each other faster.
But they are proteins and are affected by high temperatures  causes vibrations in bonds
and it loses its structure so active sites are gone. Slow down by 40°C and totally
denatured by 60.
Denatured – structural change so therefore the active site would not be a fit.
• ph – have an optimum pH, are sensitive to change, sites get denatured at wrong pH
(compartmentalization)
• [enzyme] – as [enzyme] increases, so does reaction rate up to a certain point then
stabilizes
• [substrate] – as above
* be able to recognize graphs on this
Lactase – enzyme that breaks down lactose (disaccharide). This enzyme produced by all
babies but is eventually lost by many adults. That means that these people cannot digest
milk – results in production of methane and diarrhea and flatulence → lactose intolerant.
Milk with bacterial lactase added to it can be given to these people as lactose will have
already be ‘broken down’ by the bacterial enzyme and safe for drinking – lactose free
milk. Check textbook and sheet on immobilized enzymes.
(we think an evolutionary adaptation to when we started to be farmers and keep
cows as found in Northern European people and some African tribes)
Cofactors – minerals that bind to enzymes to help maintain the shape, eg Mg, Fe, Zn, Cu
Coenzymes – vitamins that help enzymes fit the substrate better. Water-soluble enzymes
like B and C
HL
Tube sock or surgical glove – adjust to fit
Was found that some enzymes can catalyze more than one substrate. This doesn’t work
with the lock and key model. Needed a new model to explain.
Induced Fit Model
As the substrate enters the active site, it induces the enzyme to change its shape slightly
so that it fits snugly. Like a handshake where you clasp stronger to make it fit better.
Closer contact gives a higher surface area. This helps to explain how enzymes can
catalyze more than one substrate.
Enzyme inhibitors
Substances that slow down or stop the enzyme controlled reaction. Many poisons are
inhibitors and some antibiotics. There are two types;
Competitive
Similar to substrate
Attaches to active site
Blocks active site
Competes with substrate
Effect can be reduce by increasing
[substrate]
Non-competitive
Not similar
Allosteric site and not active site
Changes 3d shape and therefore active site
Substrate can’t bind
Increased [substrate] has no effect
Eg. Competitive Eg. folic acid synthesis in bacteria inhibited by Sulfonamide Prontosil
(antibiotic)
Non-competitive – Hg2+ or CN- inhibits cytochromes being oxidized. Nerve gas Sarin
blocking acetyl cholinesterase in nerve transmission.
Allosteric regulation
Controls metabolic pathways. It is a type of non-competitive inhibition that regulates
enzymes. Many enzymes have Allosteric sites – not active sites- molecules fit into these
sites and either activate or inhibit the enzyme by slightly changing the shape.
Eg. As the product builds up, the product will non-competitively inhibit the enzyme.
This whole type of metabolic pathway, where the produce will inhibit the reaction is
called product inhibition or negative feedback. – the product turns off the reaction.
Allosteric enzymes are found at the beginning of the metabolic pathway
• inhibitor is non-competitive
• binds at Allosteric site – not active site
• changes shape of the active site
• substrate cannot bind now
• done to first enzyme in pathway as this avoids a build up of intermediates
•is reversible
• pathway started when shortage of end product
eg. ATP is the inhibitor
phosphofructokinase is the enzyme
*Explain the control of metabolic pathways by end-product inhibition including the role
of allosteric sites.
Positive feedback – where the product causes the reaction to increase or turn on. Few
examples – childbirth  uterine contractions stimulate sensors and hormones that
increase the intensity and number of contractions
Jar of toothpicks – reach in with one hand and break in half in 30 sec.
Then increase [substrate], then increase [enzyme] by adding second hand, noncompetitive wear big gloves, competitive – cross some fingers