Emzymes And Biological Reactions

Question 1

What does the term metabolism refer to?

Answer 1

All the reactions of the body

Question 2

Reactions occur in sequences called metabolic pathways, these include:

Answer 2

Anabolic reactions, building up molecules e.g. Protein synthesis

Catabolic reactions, breaking molecules down, e.g. Digestion.

Question 3

What are metabolic pathways controlled by?

Answer 3

Enzymes

Question 4

General properties of enzymes?

Answer 4

They speed up reactions

They are not used up

They are not changed

They have a high turn over number. I.e. They catalyse many reactions per second

Question 5

What reactions do enzymes catalyse?

Answer 5

Reactions that are energetically favourable and would happen anyway. But without enzymes, reactions in cells would be too slow to be compatible with life

Question 6

Structure of enzymes?

Answer 6

They are proteins with tertiary structure and the protein chain folds into a spherical globular shape with hydrophilic R groups on the outside of the molecule, making enzymes soluble. Each enzyme has a particular sequence of amino acids, and the elements in the R groups determine the bonds the amino acids make with each other.

A small area with a specific 3D shape is the active site and it gives the enzyme many of its properties

Question 7

Where are the three distinct sites enzymes act?

Answer 7

Extracellular

Intracellular, in solution

Intracellular, membrane-bound

Question 8

Extracellular enzyme action:

Answer 8

Some enzymes are secreted from cells by exocytosis and catalyse Extracellular reactions.

Question 9

Intracellular, in solution enzyme action:

Answer 9

Intracellular enzymes act in solution inside cells. E.g. Enzymes that catalyse glucose breakdown in glycolysis, a stage of respiration in solution in the cytoplasm; enzymes in solution in the stroma of the chloroplasts catalyse the synthesis of glucose

Question 10

Intracellular, membrane-bound enzyme action:

Answer 10

Intracellular enzymes may be attached to membranes, for example, on the cristae of mitochondria and the grand of chloroplasts, where they transfer electrons and hydrogen ions in ATP formation

Question 11

What happens at the active sites?

Answer 11

An enzyme acts on its substrate, with which it makes temporary bonds at the active site, forming an enzyme-substrate complex. When the reaction is complete, products are released, leaving the enzyme unchanged and the active site ready to receive another substrate molecule.

Question 12

Explain the lock and key model:

Answer 12

The unique shape of the active site means that an enzyme can only catalyse one type of reaction. Other molecules, with different shapes, will not fit, ‘enzyme specificity’ means that an enzyme is specific for its substrate. This concept gave rise to the ‘lock and key theory’: the substrate is imagined fitting into the active site as a key fits into a lock. The shapes of lock and key are specific to each other.

Question 13

Explain the lysozyme and the induced fit model:

Answer 13

Observations that an enzymes shape was altered by binding its substrate suggested it was flexible not rigid, as originally thought. An alternative model, the induced fit model, was proposed, suggesting the enzyme shape alters slightly to accommodate the substrate.

A good example is the enzyme lysozyme, an antibacterial enzyme, in human saliva, mucus and tears. The active site is a groove and sugars on bacterial cell wall fit into it. The groove closes over the sugars and the lysozyme molecule changes shape around the sugars and hydrolysis the bonds holding them together. The cell wall is weakened; the bacteria absorb water by osmosis and burst

Question 14

What is the activation energy?

Answer 14

Molecules must have enough kinetic energy to approach closely enough to react. The minimum energy required for molecules to react, breaking existing bonds in the reactants and making new ones, is the activation energy.

Question 15

One way of making chemicals react?

Answer 15

To increase their kinetic energy, to make successful collisions between them more likely. Heat speeds up reactions in non living systems, but in most living organisms, temperatures above about 40*C cause irreversible damage to proteins, and they denature

Question 16

How to enzymes work?

Answer 16

By lowering the activation energy. When a substrate enters the active site of an enzyme, the shape of the molecule alters, allowing reactions to occur at lower temperatures than in the absence of enzymes

Question 17

What are the factors that affect enzyme action?

Answer 17

Environmental conditions, such as temperature and pH, change the 3D structure of enzyme molecules. Bonds are broken and the configuration of the active site is altered, changing the rate of reaction. The concentrations of enzyme and substrate also affect the rate of reaction by changing the number of enzyme-substrate complexes formed.

Question 18

The effect of temperature on the rate of enzyme action?

Answer 18

Increased temperature = increased kinetic energy of enzyme and substrate molecules and they collide with enough energy more often, increasing the rate of reaction.

Above 40*C, molecules have more kinetic energy but the reaction rate goes down because their increasing vibration breaks hydrogen bonds, changing the tertiary structure. This alters the shape of the active site and the substrate will not fit. The enzyme is denatured, a permanent change in structure.

At low temperatures, the enzyme is inactivated as the molecules have very low kinetic energy. However, the shape is unchanged and the enzyme will work again if the temperature is raised.

Question 19

The effect of pH on the rate of reaction?

Answer 19

Most enzymes have an optimum pH, at which the rate of reaction is at its highest. Small pH changes around the optimum cause small reversible changes in enzyme structure and reduce its activity, but extremes of pH denature enzymes.

The charges on the amino acid side chains of the enzymes active site are affected by hydrogen or hydroxyl ions. At low pH , excess H+ ions are attracted to negative charges and neutralise them. At high pH, excess OH- ions neutralise the positive charges. This disrupts the ionic and hydrogen bonds maintaining the shape of the active site, the shape changes, denature get the enzyme. No enzyme-substrate complexes form and enzyme activity is lost.

Question 20

Enzymes and substrate concentration:

Answer 20

The rate of an enzyme-catalysed reaction varies with changes in substrate concentration. If the enzyme concentration is constant, the rate of reaction increases as the substrate concentration increases. At low substrate concentrations the enzyme molecules have only a few substrate molecules to collide with so the active sites are not working to full capacity. With more substrate, more active sites are filled. The concentration of substrate is controlling the rate of reaction and so is a limiting factor.

As even more substrate is added, at a critical concentration, all the active sites are occupied and the rate of reaction is at its maximum, when all the active sites are full, the enzyme is saturated, even if more substrate is added, reactions cannot be catalysed any faster so the line (on the graph) plateaus. The substrate concentration is no longer controlling the rate of reaction and is so no longer a limiting factor.

Question 21

Effect of enzyme concentration?

Answer 21

As the enzyme concentration increases, there are more active sites available and therefore the rate of reaction increases

Question 22

In enzyme experiments, what must be used?

Answer 22

A buffer

It maintains a constant pH, even when the products of the reaction are acid or alkaline. Buffers may be thought of as absorbing extra hydrogen or hydroxyl ions.

Question 23

What is enzyme inhibition?

Answer 23

The decrease in rate of enzyme controlled action by another molecule, an inhibitor. An inhibitor combines with an enzyme and prevents it forming an enzyme-substrate complex

Question 24

What are competitive inhibitors?

Answer 24

Inhibitors which have a molecular shape complementary to the active site and similar to that of the substrate, so they compete for the active site.

E.g. In the mitochondrial matrix, a reaction to the Krebs cycle is catalysed by the enzyme succinic dehydrogenase

Malonic acid has a similar shape to succinic acid and so they compete for the active site of succinic dehydrogenase. Increasing the concentration of the substrate, succinic acid, reduces he effect of the inhibitor, because the more substrate molecules present, the greater their chance of binding to active sites, leaving fewer available for the inhibitor. But if the inhibitor concentration increases, it binds to more active sites and so the reaction rate is slower

Question 25

What does a non competitive inhibitor do?

Answer 25

They bind to the enzyme at an ‘allosteric site’, I.e. A site other than the active site, so they do not compete with the substrate. They affect bonds within the enzyme molecule and alter its overall shape, including that of the active site. The substrate cannot bind with the active site, and no enzyme-substrate complexes form. As the inhibitor concentration increases, more enzyme molecules denature and so the rate of reaction and final mass of product decreases. Examples of non competitive inhibitors include heavy metal ions, e.g. Lead. Some non competitive inhibitors bind reversible and some irreversibly.

Question 26

When are enzymes immobilised?

Answer 26

When they are fixed, bound or trapped on an inert metric such as sodium alginate beads or cellulose microfibrils. These can be packed into glass columns. Substrate is added to the top of the column and as it flows down, it’s molecules bind to the enzyme molecules’ active sites, both on the bead surface and inside the beads as the substrate molecules diffuse in.

Once set up, the column can be used repeatedly.

The enzyme is fixed and does not contaminate the products, the products are therefore easy to purify. Immobilised enzymes are used widely in industrial processes, such as fermentation, as they can readily be recovered for reuse.

Question 27

What is enzyme instability?

Answer 27

It’s a factor preventing the wider use of enzymes that are free in solution, organic solvents, high temperatures and extremes of ph can all denature enzymes, with a consequent loss of activity.

Immobilising enzymes with a polymer matrix makes them more stable because it creates a micro environment allowing reactions to occur at higher temperatures or more extreme pHs than normal

Trapping an enzyme molecule prevents the shape change that would denature the active site, so the enzyme can be used in a wider range of physical conditions that if it were free in solution.

Question 28

Advantages of immobilised enzymes?

Answer 28

Products are not contaminated with the enzyme

Enzymes are easily recovered for reuse

Increased stability and function over a wide range of temperature and ph than enzymes free in solution.

Question 29

Uses of immobilised enzymes?

Answer 29

Lactose free milk

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