Untitled Flashcards Set

Enzymes as catalysts

Many chemical reactions do not occur spontaneously, or they may happen very slowly

(Figure C1.1.1). In a laboratory or in an industrial process, chemical reactions may be made

to occur by applying high temperatures, high pressures, extremes of pH, by maintaining high

concentrations of the reacting molecules, or by using inorganic catalysts. If these drastic conditions

were not applied, very little of the chemical product would be formed. On the other hand, in cells

and organisms, many chemical reactions occur simultaneously, at extremely low concentrations, at

normal temperatures and under the very mild, almost neutral, aqueous conditions we find in cells.

It is the presence of enzymes in cells and organisms that enables these reactions to occur at

relatively high rates, in an orderly manner, yielding products that the organism requires, when they

are needed. Sometimes, reactions happen even though the reacting molecules are present in very low

concentrations. Enzymes are biological catalysts made of protein. They are truly remarkable

molecules. In general, catalysts:

l are effective in small amounts

l remain unchanged at the end of the reaction.

◆Enzyme: mainly

proteins (some are RNA)

that function as biological

catalysts.

◆Catalyst: a substance

that speeds up the rate

of a chemical reaction.

Catalysts are effective in

small amounts and remain

unchanged at the end of

the reaction.

Concept:

Interaction and

interdependence

Systems are based

on interactions,

interdependence

and integration of

components. Systems

result in the emergence

of new properties at

each level of biological

organization.

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C1.1 Enzymes and metabolism 381

Common mistake

It is incorrect to use the term ‘amount’ when discussing variables in practical procedures.

Use more precise terms available, such as ‘concentration’ or ‘volume’.

Figure C1.1.1 shows the benefit of increasing rates of reaction in cells. Many enzymes are always

present in cells and organisms, but some enzymes are produced only under particular conditions,

at certain stages or when a particular substrate molecule is present. By making some enzymes and

not others, cells can control what chemical reactions happen in the cytoplasm.

Role of enzymes in metabolism

There are many thousands of chemical reactions taking place within cells and organisms.

Metabolism is the name we give to these chemical reactions. These reactions can only occur in

the presence of specific enzymes. If an enzyme is not present, the reaction it catalyses only occurs

at a very slow rate. The molecules involved are collectively called metabolites. Many metabolites

are made in organisms, but others are imported from the environment, such as from food

substances, water and the gases carbon dioxide and oxygen.

water

sucrose

water

sucrose

water

sucrose

water

sucrose

water

sucrose

water

sucrose

water

sucrose

Random collision possibilities:

when sucrose and water molecules collide at the wrong angle

when sucrose and water collide at the wrong speed

for the reaction to occur, sucrose and water must collide in

just the right orientation – glucose and fructose are formed

These events are what

happens at most random

collisions.

Under normal conditions

this happens so very

infrequently it is an

insignicant event.

In the presence of one molecule of the enzyme sucrase (invertase),

approximately 3.0 × 104 molecules of sucrose are hydrolysed each

minute!

glucose

fructose

■ Figure C1.1.1 Can a reaction occur without an enzyme?

Concept:

Interaction

Interaction: The

effect or effects that

two or more systems,

bodies, substances or

organisms have on

one another, so that

the overall result is not

simply the sum of the

separate effects.

Concept:

Interdependence

Interdependence:

Biological systems are

not self-sufficient.

Molecules, cells,

organisms and

ecosystems interact

with each other

within and across

levels of organization.

The greater the level

of interaction, the

greater the degree of

interdependence.

Concept:

Interaction

Metabolism depends

on the interaction

of many different

enzymes. Enzymes

for specific reactions

are located within

compartments

(organelles) within

eukaryotic cells, for

example the enzymes

of oxidative respiration

are found within

mitochondria.

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C1.1 Enzymes and metabolism 395

Effect of enzymes on activation energy

We visualize an enzyme (E) as a large molecule that works by reacting with another compound or

compounds, the substrate (S). Initially, a short-lived enzyme–substrate complex (ES) is formed at the

active site. This complex exists at a local energy minimum and is quite stable. The transition state

(TS) is the point where there is a maximum value of energy. The transition state exists at the

top of the energy profile and is transient. Almost instantly, the product (P) is formed and the

enzyme is released unchanged. The enzyme immediately takes part in another reaction. We

represent this reaction as follows:

E + S → [ES] → ES ‡ → P + E

where ES ‡ is the transition state.

Energy is released when the ‘substrate’ becomes the ‘product’. However, to bring about the reaction,

a small amount of energy is needed initially to break or weaken bonds in the substrate, to form the

transition state. This energy input is called the activation energy (Figure C1.1.14). It is a small but

significant energy barrier that must be overcome before the reaction can happen. Enzymes work by

lowering the amount of energy required to activate the reacting molecules by providing a new,

alternative reaction pathway.

Another model of enzyme catalysis includes a boulder (substrate) perched on a slope, prevented

from rolling down by a small hump (representing activation energy). The boulder can be pushed over

the hump, or the hump can be dug away to lower it (= lowering the activation energy), allowing the

boulder to roll down and shatter at a lower level (giving products).

Energy is needed to break the bonds within the substrate. When bonds are made from the products

of an enzyme-catalysed reaction, there is an energy yield. You should be able to interpret graphs

showing this effect, for example Figure C1.1.14.

Note: virtually all enzyme-catalysed reactions still have activation energy barriers, they are just

smaller. The enzyme lowers the ‘hump’ of activation energy, it does not remove it.

products

(at lower

energy level)

triggering the fall,

either by pushing

‘boulder on hillside’ model of activation energy

or by lowering

the humpthe hump (the

enzyme way)

activation energy

free energy change

energy

reactant products

without a catalyst, this

amount of energy needs

to be put in to start the

reaction effect of catalyst

transition state

Example: sucrase

sucrose + water ⎯⎯⎯→ glucose + fructose

■ Figure C1.1.14 Activation energy

◆Activation energy:

energy required by a

substrate molecule before

it can undergo a chemical

change.

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396 Theme C: Interaction and interdependence – Molecules

ATL C1.1C

Find out more about activation energy here:

https://ed.ted.com/lessons/activation-energy-kickstarting-chemical-reactions-vance-kite

Produce a cartoon or poster, using ideas from this book and from the animation, to summarize

the role of enzymes in reducing activation energy in metabolic reactions. Producing your own

visual summaries of important biological concepts can help you understand and remember them.

16 Sketch a graph

showing the effect

of an enzyme

on the activation

energy of a

metabolic reaction.

17 Define the term

activation energy.

Top tip!

Molecules collide more frequently at higher temperatures; however, the main reason why the

reaction speeds up is that more molecules have enough energy to get over the activation energy.

Intracellular and extracellular

enzyme-catalysed reactions

Some enzymes are exported from cells, such as the digestive enzymes. Enzymes like these are put

into vesicles, secreted by endocytosis and work externally. They are called extracellular enzymes.

Chemical digestion in the gut is an example of an extracellular reaction.

However, most enzymes remain within cells and work there. These are the intracellular enzymes.

Many are found inside organelles and in the membranes of organelles, in the fluid medium around

the organelles and in the plasma membrane. Many are also in the cytoplasm (e.g. glycolysis,

page 414). Two of the main metabolic processes in respiration are glycolysis and the Krebs cycle.

These are intracellular enzyme-catalysed reactions.

18 Distinguish between intracellular and extracellular enzymes.

Generation of heat energy by the

reactions of metabolism

When glucose is oxidized to carbon dioxide and water in aerobic cell respiration, energy is

transferred from the store of chemical potential energy to heat energy (i.e. the kinetic energy of

molecular motion). This energy is no longer in store but is on the move; it is active energy. Only

part of the stored energy in a molecule is available. This is known as free energy and can be used

to do work. Reactions that release free energy are known as exergonic reactions (Figure C1.1.15).

The oxidation of glucose is an example of an exergonic reaction.

Heat generation is inevitable in metabolic reactions. Exergonic reactions involve the release of heat

because metabolic reactions are not 100% efficient in energy transfer. Mammals, birds and some

other animals depend on this heat production for maintenance of constant body temperature and

are said to be endotherms (page 766) or ‘warm-blooded’. Other animals cannot control their cell

metabolism in this way and are described as ectotherms (‘cold-blooded’).

On the other hand, reactions in which energy is absorbed, and there is more energy in the system at

the end of the reaction than at the beginning of it, are called endergonic reactions. The synthesis of

a protein from amino acids is an example of an endergonic reaction.