chapter 2:enzymes

Chapter 2:Enzymes

Higher Human

Unit 1: Cell Function and Inheritance

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Learning Intentions

• To revise purpose of enzymes• To revise how enzymes work• To examine factor affecting rate of enzyme-

catalysed reactions

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• A catalyst is a substance that:– Speeds up the rate of a chemical reaction but remains

unchanged at the end of the reaction.

• Enzymes are biological catalysts (made of protein) that are present in all living cells.

• They speed up the rate of biochemical reactions at relatively low temperatures.

• Without enzymes important biological processes like respiration and photosynthesis would proceed too slowly to maintain life.

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Biological Catalyst

Structure

• Globular proteins• One or more polypeptide

chains • Function determined by

structure• Reaction occurs at active

site• Specific for one reaction

due to shape of active site – “lock and key” hypothesis amylase

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Aim: To revise the purpose of enzymes, how they work, and examine the factors which affect enzyme catalysed reactions.

Function• Biological catalysts - increase reaction

rates• Approx 40,000 different types in human

cells• Intracellular (inside cell) e.g. catalase• Extra-cellular (outside cell) e.g. pepsin• Needed for respiration, photosynthesis,

digestion, motors, membrane pumps, receptors

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Since we have about 200 trillion cells and each one performs millions of chemical reactions, the total number of chemical reactions in the human body is about 400 billion per second every second of your life.

That's 4 times the amount of stars in our galaxy which is a mere 100 billion

How many chemical reactions happen in our body?

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Reaction Mechanism

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• Enzymes are SPECIFIC = This means that each enzyme can only react with one set of substrate molecules.

• For example amylase cannot act on glucose and potato phosphorylase cannot act on starch.

Specific Enzymes


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• The shape of a molecule of enzyme exactly matches the shape of a molecule of its substrate like a key which fits exactly into a lock. This allows the two molecules to combine briefly at a point called the active site, which brings about the reaction.

• We call this the “LOCK & KEY” theory.

Lock and Key Theory


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Model enzymes


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Model Enzymes – Synthesis reaction.



Specific Enzymes –Degradation Reaction



Factors Affecting Enzyme Activity

Enzymes are sensitive to the following:• pH• Temperature• Enzyme concentration• Substrate concentration



pH



Optimum pH• Enzymes have an optimum pH at which

they catalyse reactions best.• Most enzymes function at a pH of 5-9 with

an optimum pH of 7 (neutral).• There are exceptions to this rule:

– Pepsin secreted by stomach’s gastric glands optimum pH is 2.5 (very acidic)

– Catalase found in living cells optimum pH is 9 (very alkaline)



DenaturingEnzymes are denatured (destroyed and

inactive) at extremes of PH:

Too many H+ ions or OH+ ions will alter the shape of the enzymes active site so it can no longer fit with its substrate and catalyse a reaction.



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Pepsin

1 2 3 4 5 6 7 8 9 10 11 12

Optimum pH

Amylase

Catalase



Temperature



Optimum temperature• At low temperatures enzyme activity will be very

slow.• At warmer temperatures the enzyme activity will

increase• The temperature at which the enzyme works

best at is called its “optimum temperature”.• The optimum temperature for most animal

enzymes is 37°C. Some plant enzymes have an optimum temperature around 20°C.



Denaturing• Enzymes are denatured (destroyed and

inactive) at high temperatures (usually above 420C:

• Increasing the temperature beyond an enzymes optimum will destroy the shape of its active site so it can no longer fit with its substrate and catalyse a reaction.



Enzyme Concentration



Enzyme Concentration


• Increasing concentrations increases activity.

• Enzyme activity can be decreased in the presence of INHIBITORS (molecules which will join preferentially to the enzyme).


Inhibition

• Substrate can’t fit into active site of enzyme

• Reaction rate lowered

• 2 types: – Competitive– Non-competitive



Competitive Inhibition


• Competitive inhibitors block the active site of the enzyme molecule. This type of inhibition is reversible.


Non-competitive Inhibition


• Non-competitive inhibitors attach to some other part of the enzyme and in doing so alter the shape of the active site indirectly.

• This is irreversible inhibition.


Outcome 3

• Assessed practical • “Effect of enzyme concentration on

enzyme action”• Submit write-up by:- Mon 28th June



Substrate Concentration



Activation of Enzymes

• Many enzymes require the presence of other substrates to make them work.

• These ACTIVATORS can be mineral ions, vitamins or other enzymes. – Cofactors (coenzymes) required – metal ions – iron (Fe2+), magnesium (Mg2+), copper

(Cu2+)– organic molecules e.g. haem, NAD, coenzyme A



Inborn errors of metabolism• Metabolism – all the

chemical reactions that occur in an organism

• Error in gene coding for enzyme results in blockage in pathway

• May result in health problems



Phenylketonuria

• Occurs when enzyme required to breakdown the amino acid phenylalanine present in the diet into tyrosine.

• The excess phenylalanine is broken down into toxic substances which can lead to brain damage and poor mental development in children.

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chapter 2:enzymes

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