Biology 201 Enzymes

What are Enzymes?

Enzymes are catalysts.

A catalyst is a substance which increases the rate of chemical reaction

Catalysts themselves are NOT used up Used in living organisms

Reactions must occur at body temp and atmospheric pressure (LOW)

Without catalysts reactions would be too slow

Needed to sustain life

What are Enzymes?

Metabolism is just the cummulation of reactions 1000's of different reactions in an organism Each reaction requires a unique catalyst

These catalysts are called enzymes Enzymes act upon a substance called its substrate (reactant)

Metabolism is a chain of reactions Product of one reaction is substrate for the next A different enzyme for each step in the chain

This is called the metabolic pathway

enzyme1 enzyme2 enzyme3 enzyme4

A ---------> B ---------> C ---------> D ---------> E

How Enzymes Work

Each reaction requires a specific enzyme

Enzymes must be made of something that can take many different shapes

Proteins

There is a small part of the enzyme that contacts the substrate called the active site

Like a small cleft or indent on the surface Substrate fits into active site and interacts with exposed

amino acids by ionic or hydrogen bonding Forms enzyme-substrate complex Reaction then occurs

How Enzymes Work

This is known as the lock and key model

Specific substrate fits specific enzyme

How Enzymes Work

Induced fit model is an alternate mechanism

Enzyme's active site does not fit the substrate until the substrate actually enters the enzyme

The enzyme is not active until this happens

How Enzymes Work

Enzyme-catalysed reactions are reversible

Direction of reaction depends on concentrations

Enzymes don't effect the point of equilibrium

Enzymes only change the time to equilibrium

In cells this in not important because products become substrates for new reactions or used up

Reactions will always go forward

Added Molecules

molecules or ions other than proteins may associate with an enzyme to aid in its function

cofactor coenzyme prosthetic group

Energy Changes During a chemical reaction bonds must be broken and

new bonds are remade to form products Breaking bonds required energy endergonic Making bonds releases endergy exergonic These energies are not equal

Reaction can create a net increase of energy Reaction can require an external source of energy

Anabolic reactions build large complex molecules built up from smaller ones

These reactions are endergonic (requires energy) Protein synthesis Photosynthesis uses light energy

Energy Changes

Catabolic reactions break down large complex molecules into smaller, simpler ones

These reactions are exergonic (releases energy) Respiration releases chemical energy

All reactions begin by breaking bonds which requires an activation energy to start

Enzymes work by lowering the activation energy This is done by the enzyme-substrate complex This complex has lower activation energy Makes the reaction proceed quicker and easier

Without enzyme activation is too high to occur

Energy Changes

Enzyme Activity Factors

The activity of an enzyme is a measure of how well it catalyses its reaction

Temperature Increase temperature --> Increase kinetic energy More collisions between substrate and enzyme More successful conversions Moving higher than optimum temp reduces activity

Enzyme is made of protein High temperatures can denature the enzyme

Tertiary structure is changed 3D structure of enzyme changes too much for substrate to fit

Most enzymes in the human body have an optimum temperature around 40 C

Temperature

Enzyme Activity Factors

pH A change of pH means a change in H+ ions in surrounding Affects the bonding of R-groups Affects the shape of the enzyme Most human enzymes have optimum at 7 (neutral) Stomach enzymes (pepsin) have optimum at 2 Liver enzymes arginase has optimum at 10

pH

Enzyme Activity Factors

Enzyme concentration Enzymes are not used up during reactions Can work effectively at low concentrations Usually substrate molecules are in excess Reaction rate is limited by enzyme concentration Increased concentration --> increased reaction rate

Enzyme Concentration

Enzyme Activity Factors

Substrate concentration If substrate concentration is low some enzyme sites empty Rate of reaction will be lower If substrate conc. is then increased --> rate increases This increase continues until substrate is in excess Reaction rate approaches maximum called Vmax

Enzyme active sites are saturated with substrate

Substrate Concentration

Enzymes- quiz (open book)• 1.What is an enzyme?- identify 4 properties (4 marks)• 2. Name the two different theories that attempt to explain

how enzymes work (2 marks)• 3. Describe how each theory above works (4 marks) • 4. What do enzymes do to help a chemical reactions-

think energy. (2 marks)• 5. Name 4 factors that can effect enzyme effectiveness (4

marks)• 6. Choose two and explain how these factors effect

enzymes. Include a rough graph to support your answer (4 marks)

• 7. What word do you use to describe an enzyme that has changed shape and no longer functions (1 mark).

Co-factors & Co-enzymes

• Enzymes often need “helpers”.  • Sometimes ions or metal atoms are used. These

helpers are called cofactors (e.g. iron in haemoglobin, calcium in nerve signalling, nickel in urease etc.)

• Small molecule helpers are called coenzymes.• Coenzymes that we can't build ourselves, that

we need to get from our food in their working form, are called vitamins.  (e.g. vitamin B in respiration, vitamin C for turning genes “on”)

Co-factors & Co-enzymes

Inhibitors Inhibitors are chemicals which reduce the rate of enzyme

catalyzed reactions Alter the shape of the active site directly or indirectly

Non-reversible bind permanently disabling enzyme Permanent change of tertiary structure (break disulfide bond) Always non-competitive

Reversible inhibitors bind temporarily to enzyme Competitive inhibitors

Similar shape to substrate and fit into active site Prevents substrate from entering enzyme

Non-competitive inhibitors Bind to other parts of the enzyme altering shape of enzyme Active site no longer fits substrate

Inhibitors

Examples

Heavy metals (such as Pb, Hg, Ag and Cu).

These are non-competitive inhibitors and bid to the enzyme altering the shape. This is non-verseable

Reversible competitive inhibitors:

Antiviral drugs used to treat HIV bind to the enzymes which start DNA replications.

This stops the virus from reproducing