1. Metabolism & Enzymes

2. What I should know
There are 2 types of metabolic pathways-Anabolic and catabolic
the pathways - can have reversible and irreversible steps and alternative routes.

3. Cell metabolism
Is the collective term for all the biochemical reactions that occur in a living cell
Reactions are turned on/off or sped up/slowed down according to the cell's immediate needs and overall functions
the numerous pathways involved in building
up and breaking down cellular components
must be monitored and balanced in a
coordinated fashion
To do this, cells organise reactions into
various enzyme-powered pathways

4. Types of Metabolic Pathways
Metabolic pathways are chains of enzyme catalysed reactions in which the product of one reaction is the substrate for the next
In some cases these steps can be reversed, whereas others are irreversible
Anabolic & Catabolic Pathways
Reversible & Irreversible Pathways

5. Anabolic Pathways Synthesis reactions (building up)
Requires energy in the form of ATP to proceed
E.g. in DNA replication: DNA polymerase adds free nucleotides to the end of the newly-forming DNA, causing elongation (building up) of the new strand
E.g. Amino acids -----> Protein

6. Catabolic Pathways Breakdown reactions
Involves the release of energy to form ATP
E.g. Hydrogen peroxide (harmful) is broken down, by catalase in the liver, into Oxygen & water (harmless)
E.g. Proteins ---> amino acids

7. 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

8. Irreversible/Reversible Pathways
Glucose diffusing into a cell from a high conc outside to a low conc inside is irreversibly converted to intermediate 1 by enzyme A
Advantage: maintains a low conc of glucose inside the cell, promoting the entry of glucose
Glucose
Intermediate 1
Intermediate 2
Intermediate 3
Pyruvate
Enzyme A
Enzyme C
Enzyme B
Many enzymes

9. Irreversible/Reversible Pathways
If more intermediate 2 is produced than is required, some can be converted back into intermediate 1, (reversible)
It can then be used in an alternative pathway
E.g. to build glycogen (in animals) or starch (in plants)
Alternative routes also allows steps in a pathway to be bypassed e.g. sorbitol
The reversible and irreversible steps allow this pathway to be kept under strict and precise control
Glucose
Intermediate 1
Intermediate 2
Intermediate 3
Pyruvate
Enzyme A
Enzyme C
Enzyme B
Many enzymes

10. Do you know ?
There are 2 types of metabolic pathways-Anabolic and catabolic
the pathways - can have reversible and irreversible steps and alternative routes.

11. Control of Metabolic Pathways

12. What you should know
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

13. 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
Molecules of a substrate have an ‘affinity’ (chemical attraction) for the enzyme’s active site

14. 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

15. Activation energy
The energy required to break chemical bonds in the reacting chemicals is called the activation energy
Enzymes lower the activation energy

16. Induced Fit
Substrate molecule induces a slight change in the shape of the active site
This allows the substrate molecule to fit perfectly and change to its working conformation
The change in shape of the active site facilitates the reaction

17. Metabolic Pathways and Enzymes
Some enzymes are associated with other enzymes involved in a particular pathway to form multi-enzyme complexes
In reality, DNA/RNA polymerase aren’t just single enzymes.
Rather, they are massive multi-enzyme complexes possessed of multiple catalytic activities

18. Control by Enzymes
Metabolic reactions are controlled by the presence or absence of particular enzymes in metabolic pathways
If appropriate enzyme is present= reaction continues
If appropriate enzyme is absent= pathway stops

19. Example of control by genes
Some organisms have the ability to turn on/off genes which code for enzyme production (called ‘enzyme induction’)
It is known to occur in humans, but the process is very complicated
A good example is E.Coli, a bacteria which have the ability to turn on/off a gene to produce an enzyme in order for them to undergo respiration
Please look at pages of the TB and we’ll make notes on this example

20. Please copy Fig. 6.18 into your notes.
Underneath the diagram, make notes on the ‘Absence of Lactose’ and the ‘Presence of Lactose’
What benefit is there to the E.Coli?

21. The Effect of Substrate and End Product Concentration

22. FACTORS AFFECTING ENZYME ACTIVITY
Temperature pH
substrate concentration
enzyme concentration
inhibitors

23. INCREASING SUBSTRATE CONCENTRATION
Increasing substrate conc increases rate of reaction as more active sites become occupied
This continues until all of the active sites become occupied
further increases in substrate conc make no difference to the rate of the reaction thereafter
If displayed on a graph, the
line would increase, to a point,
where it would then level off
* See also fig in TB

24. Intra/Extracellular Signals
- Intracellular signal molecules are those that effect the cell’s metabolism & originate from within the cell
- Extracellular signal molecules effect the cell’s metabolism, but come from the cell’s environment

25. Regulating Enzyme activity
A cell has to control enzyme activity. It does this by:
Controlling number of enzyme molecules in cell
Keeping reactions (and enzymes) in compartments eg. Mitochondria or lysosome.
Changing enzyme shape (most effective)

26. Enzyme activity
Enzymes are always present due to constant expression. This means they need to be controlled.
Enzymes can be inhibited by other things binding to them

27. Competitive and Non-competitive Inhibition

28. Inhibition
Non-competitive – A molecule binds to the enzyme at another site; it changes the conformation of the enzyme and its active site. Thus, catalytic efficiency is reduced.
Competitive – A molecule with similar molecular shape to the substrate competes for the active site and reduces the concentration of available enzyme

29. 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

30. 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

31. Do you know ?
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