Higher Human Biology

Metabolism and Enzymes

Learning Intention:

To learn about the Metabolic pathways

Success Criteria:

By the end of the lesson I should be able to

• Explain the difference between anabolic and catabolic pathways.

• Describe how metabolic pathways are controlled.

Cell metabolism

• A cell's daily operations are accomplished through the biochemical reactions that take place within the cell.

• Reactions are turned on and off or sped up and slowed down according to the cell's immediate needs and overall functions.

• At any given time, the numerous pathways involved in building up and breaking down cellular components must be monitored and balanced in a coordinated fashion.

• To achieve this goal, cells organize reactions into various enzyme-powered pathways.

Cell Metabolism

• Is the collective term for all the biochemical reactions that occur in a living cell

• Many of these biochemical reactions are steps in a complex network of connected and integrated pathways that are catalysed by enzymes

Two types of metabolic pathways

• Anabolic pathways require energy and are involved in the biosynthesis of complex molecules from simpler molecules

• Catabolic pathways release energy and involve the breakdown of molecules

Control of metabolic processes

• Metabolic processes can have reversible , irreversible steps and alternative pathways which keep the process under strict control

• An example of an irreversible step- the diffusion of glucose into the cell which is converted by an enzyme into intermediate 1. This keeps the concentration of glucose in the cell low allowing for further diffusion of glucose into the cell

glucose

Enzyme B

Enzyme A

Intermediate 1

Intermediate 2

Enzyme C

Intermediate 3

Many enzyme controlled steps

pyruvate

Glycogen in animals

Starch in plants

• Reversible step- the conversion of intermediate 1 into intermediate 2 is reversible as this allows any excess to be used in an alternative pathway eg conversion of glucose to glycogen for storage

• Alternative routes bypass steps in the pathway

• Eg steps controlled by enzyme A,B and C can be bypassed when glucose is converted into sorbitol which then returns to glycolysis later in the pathway.

Metabolic pathways - Summary

• There are 2 types of metabolic pathways-Anabolic and catabolic

• the pathways - can have reversible and irreversible steps and alternative routes.

Learning Intention:

To learn about the control of metabolic pathways

Success Criteria:

By the end of the lesson I should be able to

• State that enzymes lower the activation energy required for a chemical reaction to start.

• Explain the induced fit model in relation to enzyme action.

• Explain the effect of substrate concentration on rate of the reaction.

• Explain the effect of product concentration on the rate of reaction.

• State the three types of enzyme inhibition.

• Explain the mechanisms of competitive, non-competitive and feedback (end product) inhibition.

• Metabolic reactions are controlled by the presence or absence of particular enzymes in metabolic pathways

• If the appropriate enzyme is present the reaction continues

• If the appropriate enzyme is absent the pathway stops

• Enzyme action can be regulated at the

– level of gene expression (Control of the number of

enzyme molecules)

– at a level of enzyme action (Change of enzyme shape)

Control by gene expression

Some proteins are only required at Some proteins are only required at certain times. In order to prevent certain times. In order to prevent resources being wasted, genes can be resources being wasted, genes can be switched on and off.switched on and off.

Lactose is the Lactose is the sugar found in sugar found in milk.milk.

Effect of B-galactosidase on lactoseEffect of B-galactosidase on lactose

It is made from a It is made from a molecule of molecule of glucose joined to a glucose joined to a molecule of molecule of galactose.galactose.

Jacob Monad Hypothesis- Switching genes on and off

The enzyme B-The enzyme B-galactosidase can be galactosidase can be used to breakdown used to breakdown lactose into its lactose into its component molecules.component molecules.

lactoselactose

B-B-galactosidasegalactosidase

galactosgalactosee

glucoseglucose

E.ColiE.Coli has a has a gene which gene which codes for the codes for the production of B-production of B-galactosidase.galactosidase.

BUT!! It only produces the BUT!! It only produces the enzyme when lactose is enzyme when lactose is present.present.

This is called This is called enzyme induction.enzyme induction.

Operon = Operon =

1 or more structural 1 or more structural genes with a genes with a neighbouring operator neighbouring operator gene.gene.

The operator gene controls The operator gene controls the switching on and off of the switching on and off of the structural gene. the structural gene.

operooperonn

OperatorOperator genegene

structuralstructural genegene

The operator is The operator is affected by a affected by a repressor repressor moleculemolecule. .

regulatorregulator genegene

The repressor is produced The repressor is produced under control of the under control of the regulator generegulator gene. .

RepressorRepressor moleculemolecule

Control by level of enzyme action

Enzymes

• Enzymes are biological catalysts which are essential to the maintenance of life.

• They form an enzyme-substrate complex that accelerates the rate of reaction.

Enzyme Properties

• Enzymes are globular proteins

• They possess a small region called the active site where the reaction occurs

• Enzymes are specific in the reaction that they catalyse

• Enzymes are only required in small amounts and remain unchanged at the end of the reaction

Induced Fit

• Enzyme activity conforms to the induced fit model• The substrate molecule induces a slight change

in the shape of the active site to allow the substrate molecule to fit perfectly and change to its working conformation. The change in shape of the active site facilitates the reaction.

Enzyme activation energy

•The energy required to break chemical bonds in the reacting chemicals is called the activation energy

•Enzymes lower the activation energy

FACTORS AFFECTING ENZYME ACTIVITY

• Temperature• pH• substrate concentration• enzyme concentration• inhibitors

INCREASING SUBSTRATE CONCENTRATION

• Increasing substrate conc increases rate of reaction, to a point, as more active sites become occupied

• Beyond that point, the conc of enzyme becomes limiting

INCREASING SUBSTRATE CONCENTRATION

• To increase rate of reaction beyond that point, increase enzyme conc

INCREASING ENZYME CONCENTRATION

• Increasing enzyme conc increases rate of reaction, until enzyme conc is large

• Substrate conc is now the limiting factor

INCREASING ENZYME CONCENTRATION

• More substrate must be added to increase reaction rate

Metabolic Pathways and Enzymes

• A metabolic pathway usually involves a group of enzymes

• Some enzymes are associated with other enzymes involved in a particular pathway to form multienzyme complexes

• In reality, DNA polymerase isn’t just a single enzyme. Rather, it is a massive multi-enzyme complex possessed of multiple catalytic activities

• DNA polymerase and RNA polymerase form part of multi enzyme complexes

Control of a Pathway using its enzymes

• Metabolic pathways can be controlled by the switching on or off of the first enzyme in the pathway

• If the first enzyme is switched off the rest of the pathway stops due to the lack of intermediates

• The first enzyme can be inhibited by high levels of the final product from the pathway

End-Product Inhibition• Used in the control of metabolic pathways• The end-product of the pathway inhibits the activity of

the first enzyme in the pathway• This is energetically efficient as it avoids the excessive

production of the intermediates of a pathway

Competitive and Non-competitive Inhibition

• Competitive – A molecule with similar molecular shape to the substrate competes for the active site and reduces the concentration of available enzyme

•Non-competitive – A molecule binds to the enzyme at an area other than the active site; it changes the conformation of the enzyme and its active site. Thus, catalytic efficiency is reduced.

Control of metabolic pathways - Summary

• Metabolic pathways are controlled by the presence or absence of particular enzymes in the metabolic pathway and through the regulation of the rate of reaction of key enzymes within the pathway.

• Regulation can be controlled by intra- and extracellular signal molecules.

• Induced fit and the role of the active site of enzymes including shape and substrate affinity.

• How enzymes affect activation energy. • The effects of substrate and end product concentration

on the direction and rate of enzyme reactions. • Enzymes often act in groups or as multi-enzyme

complexes. • Control of metabolic pathways through competitive, non-

competitive and feedback inhibition