Enzymes

Role of enzymes

Catalyse metabolic reactions

What is a catalyst

Speeds up reaction; not used up itself

What does it mean that enzymes are ‘specific’

They catalyse reactions involving only one type of substrate

Describe structure of an enzyme

Globular structure - contains active site (tertiary structure) to which substrate binds

What is an extracellular enzyme

Catalyses reactions outside cell

What is an intracellular enzyme

Catalyses reactions inside the cell (metabolic)

Example of an extracellular enzyme

Amylase

Describe the function of amylase

Turns starch into maltose

Where is amylase found

Released from salivary gland into mouth; released from pancreas to small intestine

Example of an intracellular enzyme

Catalase

Describe function of catalase

Prevents damage to cells by breaking down toxic hydrogen peroxide into harmless products water and oxygen

Products of catalase reaction outside breaking down hydrogen peroxide

Water and oxygen

Hydrogen peroxide formula

H2O2

Where is catalase found

Eukaryotic cells: vesicles called peroxeromes; WBCs use it when they ingest pathogens

What is a metabolite

Reactants, intermediates and products in a metabolic pathway

2 types of metabolic reactions

Catabolic and anabolic

What happens in a catabolic reaction

Large molecules are broken down to smaller ones

What happens in an anabolic reaction

Large molecules are made from smaller ones

How do enzymes make reactions happen faster

Lowers activation energy

Describe active site of enzyme

Part of tertiary structure; specific to one type of substrate

What is an enzyme-substrate complex

Formed when substrate binds to active site of enzyme

What happens to the ESC after the reaction occurs

Substrate is turned into product; enzyme-product complex; enzyme releases product

2 hypotheses for binding of enzyme and substrate

Lock and key; Induced fit

Lock and key hypothesis

Active site is specific; substrate is complementary to active site shape

Induced fit hypothesis

When ESC forms, structure of enzyme is altered; active site is made to fit better around substrate

Do all enzymes have the same optimum pH

No

Effect of pH on enzyme activity

Either side of optimum will denature enzyme

More H+ ions means more acidic or alkaline

Acidic

How is α-helix structure affected by pH

H+ attracted to -ve charges on the α-helix; ‘replace’ the bonds

Which kind of enzyme will be more affected by adding H+

Enzymes with lots of H-bonds (H+ ions replace negative charges)

What would happen to negatively charged amino acids in an active site in acidic conditions

H+ ions attracted to -ve R group; interferes with binding of substrate to active site

How do alkaline conditions affect enzymes

OH- ions may bind to +ve R groups on amino acids in active site; interferes with ESC

Effect of temperature on enzyme activity

Increases until past optimum; then denatures

Why does initially increasing temp increase enzyme activity

KE gained; move faster; more frequent collisions between substrate + active sites; more ESCs form

What is the optimum temperature of an enzyme

Temp at which enzyme catalyses at maximum rate; greatest number of successful collisions occur

Is the optimum temperature of an enzyme always 40 degrees

Usually 40 for animals; bacterial enzymes may like extremes

How enzymes in thermophilic bacteria tolerate the hot conditions

More disulfide bonds (won’t break when heated)

Why does increasing temperature past optimum reduce enzyme activity

Structure vibrates so energetically that some bonds between R groups holding active site in shape break; tertiary structure lost; enzyme denatured irreversibly

Which bonds are more likely to break in hot conditions

H-bonds, ionic

What does the temperature coefficient refer to

Increase in rate of process when temperature is increased by 10 degrees C

Formula for temperature coefficient (Q10)

Rate of reaction at (T+10) degrees / rate of reaction at T degrees

Outline how increasing enzyme concentration affects enzyme activity for fixed substrate concentration

Increase and then level out

Why does enzyme activity initially increase when increasing enzyme concentration

More active sites available; more ESCs can form per unit time

Why does increasing enzyme concentration not increase activity further after a point (for fixed substrate concentration)

Substrate conc becomes limiting factor; substrate all used up

Describe how increasing enzyme concentration affect activity at large excess of substrate / substrate continually added

Linear relationship - no limiting factor

What does enzyme concentration depend on in cells

Enzyme synthesis / degradation

How is enzyme synthesis controlled in cells

Gene upregulation/downregulation

How do cells control enzyme degradation

Continuously degrade old enzymes into amino acids and make new ones

Why do cells need to control enzyme degradation

Stop accumulation of abnormal enzymes; eliminate unneeded ones

Outline how increasing substrate concentration affects enzyme activity

Increases then levels off

Why does increasing substrate concentration initially increase enzyme activity

More frequent collisions of substrate and active site; more ESCs formed per unit time

Why does enzyme activity level out as you keep increasing substrate concentration

All enzyme molecules forming ESCs as fast as possible; all active sites occupied; enzyme conc now limiting factor

What is a cofactor

Non-protein substance required to make an enzyme controlled reaction take place at an appropriate rate

3 types of cofactors

Coenzymes, prosthetic group, inorganic ion cofactor

What is a coenzyme

Organic, non-protein molecule

How does a coenzyme work

May bind to active site to make it fit better or carry molecules between reactions; often take part in reaction and are changed in some way

What is a prosthetic group

Permanent part of an enzyme’s quaternary structure

What is an inorganic ion cofactor

Ion which increases rate of some enzymes

How do inorganic ion cofactors work

May combine with enzyme/substrate to affect charges/shape of ESC and make it form more easily

2 types of inhibitors

Competitive, non-competitive

What is an inhibitor

Substance that slows/stops enzyme controlled reaction by affecting the enzyme, influencing how substrate binds to the active site; affects enzyme’s turnover number

What is Vmax

Maximum rate of enzyme controlled reaction

What is a competitive inhibitor

Molecule with similar shape to substrate so can fit into active site

What does a competitive inhibitor form when bound to the enzyme

Enzyme-inhibitor complex; catalytically inactive

How do competitive inhibitors slow down rate of reaction

Competes with substrate for active site; prevents substrate binding; fewer ESCs can form

How to overcome a competitive inhibitor

Increase concentration of substrate; this increases chance of substrate binding to enzyme

Are competitive inhibitors reversible or irreversible

Mostly reversible; some can be irreversible

What is an inactivator

Competitive inhibitor which binds irreversible

Can enzyme reach VMax with competitive inhibitors present

Yes, if you increase substrate concentration - the curve will just be less steep

Where do non-competitive inhibitors attach

Allosteric site

What is an allosteric site

Somewhere not the active site

How do non-competitive inhibitors work

Attach to allosteric site, distorting tertiary structure and active site. Fewer ESCs form per unit time, slowing RoR

Effect of increasing substrate concentration with non-competitive inhibitors

No effect

Can VMax be reached with non-competitive inhibitors

No

What does level of inhibition depend on for Non-competitive inhibitors

Number of inhibitor molecules present

Are non-competitive inhibitors reversible or irreversible

Can be either