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UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes

Enzymes: 1. Introduction - what are enzymes? what do they do? why are enzyme reactions so important to the biochemistry of living organisms?

Doc Brown's Biology exam study revision notes

There are various sections to work through, after 1 they can be read and studied in any order.

Sub-index of biology notes on enzymes and digestion


1. Introduction - metabolism, what are enzymes? why are they so important to living organisms?

Examples of biological molecules mentioned in conjunction with enzymes

Carbohydrates like sugars and starch are a combination of the elements carbon, hydrogen and oxygen (C, H and O).

Lipids (vegetable oils/animal fats) are mainly a combination of the elements carbon, hydrogen and oxygen (C, H and O), though sometimes phosphorus (P) too.

Proteins are mainly a combination of the elements carbon, hydrogen, nitrogen and oxygen (C, H, N and O), sometimes sulfur (S) too, enzyme proteins might contain metal ions like Zinc Zn2+ which form part of the active site.

Enzymes are involved in all the reactions to synthesise these larger molecules or break them down into smaller molecules.

Metabolism is the scientific term used for all the chemical reactions that go on inside an organism's body - mainly cell chemistry.

Metabolic reactions:

(i) synthesise molecules of all shapes and sizes for specific uses in an organism,

(ii) break larger molecules down into smaller ones (e.g. in digestion),

(iii) supply the energy needs for the chemistry of every cell from respiration in the mitochondria.

Most metabolic reactions are controlled by specific enzymes.

What are enzymes? and why are they so important in living systems?

Enzymes are complex protein molecules made of chains of linked amino acids that catalyse most chemical reactions that go on in cells.

Each of these biological catalysts has a unique complex 3D protein structure, particularly the shape of the active site, into which the specific substrate molecule's shape 'fits in' to give the enzyme substrate complex, and be chemically changed (key and lock mechanism).

In forming this enzyme-substrate complex, the enzymes provide a chemical reaction pathway of lower activation energy, so, for any given temperature, a greater proportion of reactant molecules have sufficient kinetic energy to change when they collide with the active site on the enzyme.

The activation energy for an enzyme catalysed reaction (top of the green hump shown on the diagram above) is the minimum kinetic energy the reactant molecules must have to break bonds and undergo a chemical change.

The black higher activation energy curve represents the non-catalysed reaction pathway.

For more details see GCSE chemistry - rates of reaction - effect of a catalyst

Because of their unique molecular structure, each enzyme catalyses a specific reaction (rarely catalyse more than one reaction).

Most enzyme catalysed reactions involve:

Breaking large molecules down into smaller ones.

Building small molecules into large molecules.

Overall, always changing one molecule into another.

(see later in the details of 'key and lock' mechanism theory).

Reminder - catalysts are substances that increase the speed of reactions by lowering the activation energy needed, BUT, they are not chemically changed overall or been used up after the reaction has taken place.

Most biochemical reactions have and require, a specific enzyme to catalyse it.

This is referred to as the specificity of an enzyme.

Enzymes are true biological catalysts, speeding up reactions without being used up in the chemical processes they facilitate.

Every protein an organism requires, including enzymes, is coded for by a different gene in the DNA genome.

This specific gene not only determines the sequence of the amino acids in an enzyme protein, but also its unique shape, and it is the shape that largely determines what an enzyme can do.

The enzyme shape is created by the folding and coiling of chains of amino acids joined together in the protein molecule.

The 3D shape of the enzyme determines the unique 3D shape of the active site.

Only one type of molecule can fit into the active site - if denatured, it becomes inactive.

The rate of chemical reactions are increased by increase in temperature, but higher temperatures may harm the structure and function of complex biological molecules like the enzyme protein molecules.

Therefore the catalytic power of enzymes speeds up the thousands of different chemical reactions which enable most organisms to live specifically at relatively low temperatures - in fact, to keep them alive at any temperature!

Extremophiles e.g. like bacteria growing near hot volcanic undersea hydrothermal vents still have enzymes but their activity is much less than in organisms at the typical lower temperatures of the Earth's surface.

Without enzymes there would be no life - no photosynthesis in plants, no protein synthesis and respiration in plants and animals, so without these processes there would be no life!

e.g. mitochondria contain all the enzymes needed for the chemical reactions involved in respiration - the source of energy to power cells,

and plant cells have all the enzymes needed for photosynthesis - chlorophyll (not an enzyme) is just one molecule in the many required for the process.

Moreover, all the chemical processes of life must well controlled to keep things in balance so any organism can function properly.

This involves enzymes and hormones for example e.g. the right levels of sugar in the blood (hormone control), what the organism can do with the sugars (enzyme controlled).

Hormones may control the appropriate concentrations of substrates and products as well as temperature - all important variables that need controlling.

BUT, if the rates of so many chemical reactions are not controlled in harmony with each other, then cells may be damaged beyond repair hence endangering the whole organism,

AND many of the reactions involved are facilitated by enzymes.

All of the descriptions of enzyme action on this page are greatly simplified, and most biochemical reactions are multi-stage process i.e.

initial substrate reactant 1 == enzyme 1 ==> product 1  == enzyme 2 ==> product 2 etc. == last enzyme ==> final product

so, please remember this when studying my 'pretty' simplified diagrams!


Summary of learning objectives and key words or phrases

Be able to explain what an enzyme is and they do as biochemical catalysts.

Be able to draw and understand a diagram showing the profile for an uncatalysed reaction, and catalysed reaction, and showing the 'hump' of the activation energy and its significance.

Using a diagram for an enzyme reaction explain the importance of lowering the activation energy for the biochemistry of living organisms.


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INDEX of biology notes on enzymes and digestion

(Enzymes are also dealt with in my GCSE chemistry notes chemistry - biotechnology)

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