1.
Most organisms are active in a limited temperature range
Identify the role of enzymes in metabolism, describe their chemical composition and use a simple model to
describe their specificity on substrates
Enzyme action
• Enzymes - protein molecules acting as biological catalysts, increase the rate of the reactions that occur in living organisms
• Intracellular enzymes are used within the cells that produce them (e.g. enzymes in cellular respiration and photosynthesis)
• Extracellular enzymes act outside the cells that produce them (e.g. digestive enzymes)
• While enzymes participate in reactions as biological catalysts they are not used up and are available for reuse (the activation
energy required to start the reaction is lower when an enzyme is present)
Enzyme structure
• Enzymes are made up of proteins and the basic building block of proteins is the amino acid. Two amino acids bonded together
form a dipeptide. When a number of dipeptides join together a polypeptide chain is formed. Polypeptides form proteins. These
chains fold in a specific way forming active sites
• Many enzymes require the presence of other factors as well as the protein part before they act. These non-protein parts are
called cofactors and include metallic ions like iron, calcium, copper and zinc. If the cofactor is an organic molecule like a vitamin
it is called a coenzyme
Figure 9.2.1.1 - (a) From a pool of amino acids
form a dipeptide when joined by a peptide bond. (c)
A polypeptide is formed when many peptide bonds
are formed. (d) Polypeptides fold into specific
shapes to act as enzymes. Each has its own
specific active site that combines with its substrate
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Enzymes and their substrates
• The compound acted on by an enzyme is called a substrate
• The compounds obtained as a result of the enzymes action on the substrate are called the products
• Enzymes are highly specific in their action – each enzyme acts upon a particular substrate
• The shape of an enzyme at a region, its active site fits with part of the substrate molecule – this is called the lock & key model
• In some cases the active site of the enzyme varies slightly from that of the substrate and the two fit only after contact when the
substrate induces a complementary shape at the enzyme’s active site – this is called the induced fit theory of enzyme action
• Poisons like cyanide and arsenic work by blocking the active sites of enzymes and stopping their action
Figure 9.2.1.2 – Each enzyme only reacts with one kind of substrate. The active site of each enzyme is able to bind to part of the
specific substrate. Two models of enzyme action are shown: (a) ‘lock and key’ model; (b) ‘induced-fit’ model
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The essential role of enzymes in metabolism
• There are hundreds of chemical reactions taking place in human body cells every second, most of which would never take
place at the temperature and the pH of living things unless they are catalysed by enzymes
• Example – respiration: glucose is oxidised and the energy stored in its bonds is released as ATP. Without enzymes this
reaction has a very high activation energy reached only at very high temperatures. If the reaction takes place at high
temperatures there are two main disadvantages:
all the energy is released spontaneously and is lost to the cell as it cannot be trapped
high temperatures can damage living molecules
with enzymes present the activation energy is reduced and the reactions can take place at moderate temperatures
• Enzymes catalyse steps in metabolic pathways and work in teams to produce an end product needed by the organism. Every
enzyme plays an essential role in the process. If one enzyme is missing or defective then the entire pathway is affected
Identify the pH as a way of describing the acidity of a substance
• The pH of a solution is the measure of the concentration of hydrogen ions per litre of the solution
• The pH of a neutral solution, like water is 7.0. A pH below 7.0 indicates an acid – the lower the pH the more acidic the solution.
•
A pH higher than 7.0 indicates a basic solution
Most biological fluids have a pH between 6 and 8 (e.g. blood pH is maintained at about 7.4). However there are a few extremes
like gastric juice which has a pH of about 2
PRACTICAL
Identify data sources, plan, choose equipment or resources and perform a first-hand investigation to test the
effect of:
increased temperature
change in pH
change in substrate concentrations
on the activity of a named enzyme
Renin is an enzyme found in the stomach of calves and in junket tablets – it causes the proteins in milk to set into semi-solid curds
(ii)
Aim: To demonstrate the effect of temperature, pH and
substrate concentration on the ability of renin to solidify milk
Part 1 – Temperature
(i)
Measure and mark 2.5cm on 4 large test-tubes
containing 0.01g of junket powder
(ii)
Warm or cool 4 samples of milk in a beaker to
5oC, 40oC, 90oC & room temperature
(iii)
Add the milk to the mark on the test-tubes
containing 0.01g of junket powder
(iv)
Add one test-tube to a water bath at 5oC,
another at 40oC and another at 90oC and leave
the fourth as a control at room temperature
(v)
Time how long it takes for the milk to set
Results
Temperature (oC)
Control (room temp)
5oC
40oC
90oC
Time to set
Over 5 min
Over 5 min
1 min 48 sec
Over 5 min
Part 2 – pH variation
(i)
To three preheated test-tubes containing 2.5cm
of milk and 0.01g of junket powder in a 40oC
water bath. To one add 2mL of HCl (acid), 2mL
of NaOH (base) to another, and the third as a
control with no pH alteration
(ii)
Time how long it takes for each to set
Results:
pH
Acidic
Alkaline
Control
Time to set
Over 5 min
Over 5 min
1 min 48 sec
Part 3 – Substrate concentration
(i)
Dilute a milk sample with water, add 0.01g
of junket powder, immerse in a 40oC water
bath and time how long it takes to set
Results:
Compare this time with the time normal
milk takes to set at the same temperature
Sample
All milk sample
Diluted milk sample
Time to set
1 min 48 sec
Over 5 min
3
EXPLANATION:
• The rate of an enzymic reaction is affected by several factors including temperature, pH and substrate
concentration
Temperature:
• At high temperatures, enzymes are permanently denatured (their
structure is permanently changed and even when the temperature
returns to normal, they remain inactive as their active site is damaged
and the protein molecule is unwound)
• Enzymes that are inactivated in low temperatures become active
when the temperature returns to normal
• Most human enzymes have an optimal temperature of about 37oC
(normal body temp)
• Heat-tolerant organisms like bacteria living in hot springs have
enzymes with high optimal temperatures
pH:
• Each enzyme has an optimal pH at which it acts best
• A change in pH can change the shape of an enzyme’s active site affecting its ability to
combine with the substrate. Because the enzyme is less able to combine with its
substrate it is unable to act and metabolic reaction declines
• Enzymes becomes less efficient if the variable value is greater or less than optimal
• Carbonic anhydrase – found in human blood has optimal pH of 7.4 (bloods normal pH)
• Pepsin – found in human stomachs has an optimal pH of 2 (acidic – gastric juice)
• Trypsin – found in the human small intestine has an optimal pH of 8.0 (basic)
Substrate concentration:
• The addition of more substrate to an enzyme solution will initially increase the rate of reaction if not all of the
active sites of the enzyme present are occupied but then plateau
• The solution contains a set amount of enzyme and if no more is added the rate of reaction declines as all of
the active sites of the enzyme molecules become occupied
Enzyme concentration:
• Only a very small number of enzyme molecules are usually involved in a reaction and these produce a given
amount of product per unit time. If the amount of enzyme is increased, the amount of product made per unit
time increases but amount of product will still be limited by the amount of substrate
• Enzyme molecules are not used up in a reaction and are available for reuse
Inhibition:
• Other molecules may compete with the normal substrate for active sites of enzymes and this compound may
combine with the active site interfering with normal substrate-enzyme reactions and inhibiting formation of
the normal product
• Enzyme inhibition can cause death
• Cyanide acts by inhibiting cytochrome c oxidase, an enzyme important in aerobic respiration (provides ATP
needed for life) and for this reason is extremely poisonous
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Explain why the maintenance of a constant internal environment is important for optimal
metabolic efficiency
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The external environment can vary greatly. In spite of this living cells can exist in a relatively unchanging
stable environment
In healthy people, whether they are eating or fasting their blood glucose is kept within 3.6 - 6.8 mmol/L and
regardless of weather conditions their core body temperature is kept around 37oC
Multicellular organisms have mechanisms that enable an enzyme to operate at its optimal capacity by
providing an environment that has relatively constant temperature, pH and substrate concentration. Since
the activity of enzymes influences the outcomes of metabolism, constant internal environmental conditions
equate to optimal enzyme activity and metabolic efficiency
The internal environment of living cells in the human body is a liquid consisting of tissue fluid (liquid that
surrounds and bathes the membranes of nearly all cells), plasma (the liquid part of the blood in which blood
cells are suspended) and other fluids
Describe homeostasis as the process by which organisms maintain a relatively stable internal
environment
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Homeostasis is the condition of a relatively stable internal environment, maintained within narrow limits
When deviation occurs mechanisms act to restore values to the ‘normal’ state
Factors like infection, trauma, exposure to toxic substances or extreme conditions such as immersion in icy
water, auto-immune diseases and inherited disorders may lead to a failure of homeostasis and is potentially
life threatening
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Figure 9.2.1.3 – Summary of major variables that are subject to homeostasis in mammals
Variable
Temperature
Blood glucose
Water
Normal Range
36.1oC – 37.8oC
3.6 – 6.8 mmol per L
Daily intake must
balance daily loss
Ions (e.g. plasma Ca2+)
pH of arterial blood
Blood pressure – arterial
Diastolic (relaxed)
Systolic (contracted)
Urea (nitrogen containing
wastes) in plasma
Red blood cells (contain
haemoglobin)
2.3 – 2.4 mmol per L
7.4
13.3 kPa (1000 mm Hg)
5.33 kPa (40 mm Hg)
< 7 mmol per L
Haemoglobin values:
Females – 135g per L
Males – 150g per L
Comments
Temperature of internal cells of the body is called the core temperature
Blood glucose is typically maintained within narrow limits regardless of diet
Body tissues vary in their water content. Bone contains about 20% water and b
about 80% water. In prolonged dehydration, fluid moves from cells and tissue f
into blood
Specific ions are required by some tissues
This pH is necessary for enzyme action and nerve cells
Transport of blood depends on maintaining adequate blood volume and pressu
Waste products of cellular processes must be removed by the kidneys to preve
toxic effects on cells
Essential for transport of oxygen - Erythropoietin, a hormone produced by the
kidney, acts on red bone marrow and stimulates red blood cell production
Explain that homeostasis consists of two stages:
detecting changes from the stable state
counteracting changes from the stable state
Stage 1 - Detecting changes from the stable state
• In this stage a sensor of some kind detects a
change in a specific variable from the desired
stable level
• The fact that there has been an undesirable
change is then transmitted to the next part of the
control system
Stage 2 - Counteracting changes from the stable
state
• An effector receives the message that there has
been an undesirable change that must be
counteracted and the variable is restored to its
desired level (this is a negative feedback
mechanism)
• In positive feed back mechanisms this stage
varies. While in negative feedback mechanisms
the process corrects deviation positive feedback
mechanisms causes the system to reinforce the deviation and change it further (positive feedback
mechanisms are rare though they do exist - e.g. entry of Na+ into neurons, the entry of one ion stimulates
entry of more Na+)
Figure 9.2.1.4 – Detecting and counteracting change. A diagrammatic summary of the two interrelated stages of
homeostasis. Note that their action relies on negative feedback systems. If a variable slightly overshoots the
optimal as a result of effector action, the counter negative feedback system will respond to correct the overshoot.
These actions occur continuously in the body so that optimal levels of variables are continually fine-tuned
Body systems contribute to homeostasis
• Various mechanisms monitor conditions inside the body and, when change is detected, body systems react
to restore the balance. In humans, cells form tissues and systems that play an essential role in homeostasis.
With the exception of the reproductive system, all body systems play a part in homeostasis
Figure 9.2.1.5 – Summary of the contribution of some body systems to homeostasis
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The hormonal and nervous systems are the major systems responsible for the control and coordination of
homeostasis