1.1 identify the role of enzymes in metabolism, describe their chemical composition and use a simple model to describe their specificity on substrates

EnzymesEnzymesEnzymes are proteins composed of amino acids (see graphic at right) which act as biological catalysts ie they speed up chemical reactions without being changed or used up themselves.

Enzymes reduce the amount of energy needed to start a chemical reaction (ie they lower the activation energy), thus they control the rate of chemical reactions in the body.

Metabolism is the set of chemical reactions that occur in living organisms to sustain life. Metabolism depends on enzymes as the reactions would be too slow to maintain life without them.

Metabolism is usually divided into two categories. CatabolismCatabolism breaks down organic matter, for example to harvest energy in cellular

respiration. AnabolismAnabolism uses energy to construct (building up) components of cells such as proteins

and nucleic acids.

Naming EnzymesNaming EnzymesDifferent kinds of enzymes are named in different ways.

Enzymes are often named by adding a suffix 'ase' to the root word of the substrate (chemical the enzyme acts on) eg Lipase (breaks down fats), Sucrase (breaks down sucrose).

Sometimes the enzymes are named on the basis of the reaction that they catalyse eg Polymerase (aids in polymerisation), Dehydrogenase (removal of H atoms ie “de-hydrogen-ing”).

Some enzymes have been named based on the source from which they were first identified eg Papayin from papaya.

The names of some enzymes ends with an 'in' indicating that they are basically proteins eg Pepsin, Trypsin etc.

The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes. Each enzyme controls a specific reaction in a metabolic pathway.

How do enzymes catalyse metabolic reactions?How do enzymes catalyse metabolic reactions?The globular protein composing an enzyme is folded to create an active site for temporarily binding the substrate (chemicals to be broken down or combined). If the active site is misshapen then the enzyme will fail to catalyse the reaction. Poisons such as arsenic and cyanide block the active site of enzymes and stops their action.

There are two modelstwo models used to describe how an enzyme works, and to explain its specificity. In each model, once the enzyme has catalysed the reaction it is released unchanged and is free to be reused.

The lock-and-keylock-and-key model model shows how each enzyme is specific for a specific substrate. The active site of the enzyme molecule has a specific shape which fits onto the substrate, forming an enzyme-substrate complex. Only a specific substrate(s) can bind in that active site which makes the enzyme specific to that substrate. (animation)

The induced fitinduced fit model model, a more recent modification on the lock-key model, proposes that the active site slightly changes its shape to accommodate the substrate perfectly. Analysis of the shape of molecules shows that the active site is more flexible than a “key-hold”, and can slightly alter its shape to fit more closely with the substrate. (animation)

Many enzymes require the presence of cofactorscofactors (eg metallic ions such as iron, calcium, copper, zinc, potassium and magnesium) to produce the correct active site shape. If the cofactor is an organic molecule, such as a vitamin, it is called a coenzymecoenzyme.

Many vitamin deficiency diseases occur due to insufficient cofactor preventing normal metabolism.

Common Enzymes and their functionCommon Enzymes and their function

Enzyme nameEnzyme name FunctionFunctionLactase Breaks down the sugar lactose (milk sugars)Sucrase Breaks down the complex sugars and starchesMaltase Breaks down the sugar maltose into glucoseAmylase Breaks down starch to glucoseProtease breaks down proteins in meats, nuts, eggs, and cheesePepsin breaks down proteins into peptidesPeptidase breaks down small peptide proteins to amino acidsTrypsin derived from animal pancreas, breaks down proteinsPapain derived from raw papaya, works well breaking down small and large

proteins (component of meat tenderiser powder)Lipase breaks down fats (lipids) found in most dairy products, nuts, oils, and

meatCellulase breaks down cellulose, plant fibre; not found in humansCatalase Breaks down hydrogen peroxide waste produced in cells

Factors that affect enzyme activityFactors that affect enzyme activityEnzymes are sensitive molecules and often have a narrow range of conditions under which they operate properly. Conditions which denature (permanently change the shape of) a protein will prevent an enzyme functioning if it alters the active site. Can you think of some conditions which might affect the shape of a protein?

The rate of an enzyme reaction (and thus the rate of metabolism) is affected by several factors, including the presence of cofactors, the presence of inhibitors, enzyme concentration, substrate concentration, temperature, and pH.

We will perform a series of mandatory experiments to investigate the effect of the last 3 factors on enzyme activity.

Effect of enzyme concentrationEffect of enzyme concentrationDescribe the change in the rate of reaction when the enzyme concentration is increased (assuming that there is plenty of substrate present).

Suggest how a cell may vary the amount of enzyme present in a cell.

Effect of substrateEffect of substrate concentrationconcentrationDescribe and explain what is being shown by the graph, which assumes a fixed amount of enzyme and ample cofactors:

The addition of more substrate will initially increase the rate of reaction if not all active sites of the enzyme present are occupied. However, the rate of reaction plateaus (reaches a maximum) as all the active sites are occupied, so adding more substrate will not increase enzyme activity further.Effect of temperatureEffect of temperatureDescribe and explain what is being shown by the graph:

The addition of heat causes more collisions between enzyme and substrate, thus increasing rate of reaction until the optimum temperature is reached, providing the maximum rate of enzyme activity. The optimum temperature will depend on the organism –for humans it is close to 37oC. Once temperature rises above the optimum, the enzyme is denatured (protein shape is permanently changed) and rate of reaction falls dramatically until it no longer active.

TASK:TASK:Use this graph to compare the range and the optimum temperature for the enzymes from humans and a type of thermophilic bacteria.

The range of activity for human enzymes is between 0 - 42oC, while the range of activity for the thermophile is between 0 – 66oC.

The optimum temperature for human enzymes is 37oC while the optimum for the thermophile is around 55oC.

Effect of pHEffect of pHThe pH of a solution is a measure of the concentration of Hydrogen ions, which determines the acidity of a solution. A pH of less than 7 is acidic, pH of more than 7 is basic, while pH of 7 is neutral.

Describe and explain what is being shown by the graph: As pH changes from acidic (pH<7) to neutral (pH 7), the enzyme activity increases until a maximum at pH 8 (= optimal pH). Enzyme activity decreases as pH increases (becomes more basic) from the optimum.TASK:TASK:Suggest why pepsinpepsin, an enzyme found in the stomach, is not active at pH 8. What would happen to the activity of pepsin if we significantly raised the pH of the stomach contents using antacids?

Saliva has a pH of between 6 and 7.4. Which colour on the graph represents the activity of the salivary enzyme amylase? Which colour represents trypsin, found in the intestines?

1.3 explain why the maintenance of a constant internal environment is important for optimal metabolic efficiency

The importance of a constant internal environmentThe importance of a constant internal environment

Living organisms made of cells, which must function efficiently to maintin life. All chemical reactions within cells must occur efficiently and be effectively coordinated to bring abut optimal metabolic efficiency.

In order to achieve this, the internal environment of an organism must be maintained within a narrow range of conditions, eg temperature, volume and chemical content, so that enzymes can function effectively and metabolic efficiency can be maintained.

Homework Task:Homework Task:Read over Unit 5.1 NSW Biology Chapter 5 page 285 onwardsRead over Unit 5.1 NSW Biology Chapter 5 page 285 onwards

Complete from Ch 5 NSW Text section 5.1 Review on page 294 Questions 1-5 and 8 to 10 for homework.