2.4- Enzymes

What is an enzyme

• a biological catalyst that speeds up rate of reaction by lowering activation energy

• remain unchanged and can be re used

• activity affects both structure and function within cells, tissues and organs

structure of enzymes

• long chains of amino acids

• folds into specific shape (tertiary structure) that determines shape of active site

• globular protein

• temporary bonds formed with substrate to form enzyme-substrate complex

anabolic reaction

• bond making

• two substrate molecules held closely together to reduce repulsion so they can bond easily

catabolic reaction

• bond breaking

• fitting in active site puts strain on bonds so molecule breaks up easily

intracellular enzymes

work within cells and organelles

example of intracellular enzyme

• catalase breaks down hydrogen peroxide

• 2H₂O₂→ 2H₂O + O₂

what is a metabolic pathway

a sequence of enzyme catalysed reactions where the product of one reaction is the substrate of the next

extracellular enzymes

some enzymes are secreted from cells where they are made and act on substrates outside of the cells

example of extracellular enzyme

• amylase secreted from salivary glands acting in the mouth and secreted from pancreas acting in small intestine

• trypsin made in pancreas and acts in small intestine

what is a cofactor

a non-protein molecule that helps catalyse reactions

types of cofactor

• organic

• inorganic

what is a cosubstrate

ions that bind to substrate to form correct shape to bind to active site of enzyme

coenzymes

• type of cofactor

• organic non-protein molecules

• often bind temporarily to active site just before/ as substrate binds

• coenzymes changed during reaction→ need to be recycled into original state

prosthetic groups

• type of cofactor

• inorgnic non-protein molecule

• permanently bound to an enzyme

prosthetic group example

• zinc ion permanently bound to carbonic anhydrase

other cofactors

• inorganic non protein molecules e.g. mineral ions

• not permanently bound to enzyme

• temporarily associate with either the substrate or the enzyme during reaction to increase the rate of reaction

effect of cofactors on structure + e.g.

• some cofactors change charge or distribution of charge on surface of either substrate or enzyme

• makes the enzyme-substrate complex easier to form

• e.g. amylase only functions if chloride ions are present

lock and key model of enzyme action

• the substrate fits into the enzyme exactly in the same way that a key fits into a lock

• the active site and substrate have a complementary shape

induced fit model of enzyme action

• active site and enzyme are not completely complementary- active site is not a fixed shape

• active site changes shape to form enzyme substrate complexes→ flexible

effect of enzymes on activation energy

• puts strain on substrate bonds, weakening them

• lowers activation energy required as less energy required to break bonds

factors affecting rate of enzyme controlled reaction

• temperature

• pH

• substrate concentration

• enzyme concentration

• concentration of inhibitors

effect of temperature on enzyme controlled reactions up to optimum temp

• higher temp=particles have higher Ek= more frequent collisions= more successful collisions= more enzyme substrate complexes formed 

optimum temperature

temperature at which rate of reaction is at its maximum

effect of temp on enzymes past optimum

• hydrogen bonds begin to break down

• tertiary structure of protein changes

• shape of active site changes→ no longer complementary to substrate

• no ESC formed so reaction stops

temperature coefficient

the increase in rate of process when temp is increased by 10°C

Q₁₀=rate of reaction at (T+10)°C/rate of reaction at T°C

effect of pH on enzyme activity

• each enzyme has an optimum pH

• if lower than optimum pH, excess H⁺ ions disrupt H- bonds and ionic bonds→ impacts shape of active site

• H⁺ ions will alter charges on amino acids in active site → interferes with binding of substrate to active site

• same above above optimum pH with OH^- ions.

calculating pH

pH=-log₁₀ [H⁺]

what is a buffer

• a substance that resists a change in pH

• accept or donate protons to maintain pH

effect of substrate concentration on enzymes

• lower substrate concentration:

  • too few substrates to occupy all available active site

  • enzymes have lowered chances of successful collisions

• intermediate substrate concentration:

  • more active sites occupied at any one time

  • higher rate of reaction→ V_max

• higher substrate concentration:

  • no effect as all active sites full

  • enzyme concentration is a limiting factor

effect of enzyme concentration on enzymes

• low enzyme concentration:

  • few enzyme molecules to allow all substrate molecules to find an active site

• intermediate enzyme concentration:

  • all substrate molecules can occupy an active site at once

  • rate of reaction increased to V_max

• High enzyme concentration:

  • no effect→ all substrates being catalysed

  • substrate concentration is a limiting factor

types of inhibitors

• competitive

• non-competitive

competitive inhibitors

• usually not permanent→ reversible

• in competition for active site→ if substrate concentration is increased, inhibitor effect is reduced

non-competitive inhibitors

• usually not permanent→ reversible

• binds to allosteric site→ changes shape of active site, preventing ESCs forming

effect of competitive inhibitors

overcome by increasing substrate conc.

effect of non-competitive inhibitors

not affected by increasing substrate conc.

non-reversible inhibitors

• bind permanently to enzyme

• strong covalent bonds formed between enzyme and inhibitor

end product inhibition

• allows control of metabolic pathways so there isn’t too much or too little substance in cells

• end product acts as an inhibitor of an enzyme earlier in the pathway