lock and key hypothesis gcse biology

Lock-and-key model

strong>Lock-and-key model n., [lɑk ænd ki ˈmɑdl̩] Definition: a model for enzyme-substrate interaction

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Lock-and-key model Definition

Lock-and-key model is a model for enzyme-substrate interaction suggesting that the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. In this model, enzymes are depicted as highly specific. They must bind to specific substrates before they catalyze chemical reactions . The term is a pivotal concept in enzymology to elucidate the intricate interaction between enzymes and substrates at the molecular level. In the lock-and-key model, the enzyme-substrate interaction suggests that the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. Like a key  into a  lock , only the correct size and shape of the substrate ( the key ) would fit into the  active site  ( the keyhole ) of the enzyme ( the lock ).

Compare: Induced fit model   See also: enzyme , active site , substrate

Lock-and-key vs. Induced Fit Model

At present, two models attempt to explain enzyme-substrate specificity; one of which is the lock-and-key model , and the other is the Induced fit model . The lock and key model theory was first postulated by  Emil Fischer   in 1894. The lock-and-key enzyme action proposes the high specificity of enzymes. However, it does not explain the stabilization of the transition state that the enzymes achieve. The induced fit model (proposed by Daniel Koshland in 1958) suggests that the active site continues to change until the substrate is completely bound to the active site of the enzyme, at which point the final shape and charge are determined. Unlike the lock-and-key model, the induced fit model shows that enzymes are rather flexible structures. Nevertheless, Fischer’s Lock and Key theory laid an important foundation for subsequent research, such as during the refinement of the enzyme-substrate complex mechanism, as ascribed in the induced fit model. The lock-and-key hypothesis has opened ideas where enzyme action is not merely catalytic but incorporates a rather complex process in how they interact with the correct substrates with precision.

Key Components

Components of the lock and key model:

• Enzyme : the enzyme structure is a three-dimensional protein configuration, with an active site from where the substrate binds.
• Substrate : often an organic molecule, a substrate possesses a structural feature that complements the geometry of the enzyme’s active site.

In the lock and key model, both the enzymes and the substrates facilitate the formation of a complex that lowers the activation energy needed for a chemical transformation to occur. Such reduction in the activation energy allows the chemical reaction to proceed at a relatively faster rate, making enzymes crucial in various biological and molecular processes.

Lock-and-key Model Examples

Some of the common examples that are often discussed in the context of the Lock and Key Model are as follows:

• Enzyme lactate dehydrogenase with a specific active site for its substrates, pyruvate and lactate. The complex facilitates the interconversion of pyruvate and lactate during anaerobic respiration
• Enzyme carbonic anhydrase with a specific active site for the substrates carbon dioxide and water. The complex facilitates the hydration of carbon dioxide, forming bicarbonate
• Enzyme lysozyme binding with a bacterial cell wall peptidoglycan, which is a vital immune function

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• Aryal, S. and Karki, P. (2023).  “Lock and Key Model- Mode of Action of Enzymes”. Microbenotes.com. https://microbenotes.com/lock-and-key-model-mode-of-action-of-enzymes/
• Farhana, A., & Lappin, S. L. (2023, May).  Biochemistry, Lactate Dehydrogenase . Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK557536/

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Principles of Organisation

• Cells are the basic building blocks of all living organisms.
• A tissue is a group of cells with a similar structure and function.
• Organs are aggregations of tissues performing specific functions.
• Organs are organised into organ systems, which work together to form organisms.

The digestive system is an example of an organ system in which several organs work together to digest and absorb food.

A catalyst is a substance that speeds up the rate of a chemical reaction but is not itself changed by the reaction. An Enzyme is a biological catalysts that speed up the rate of chemical reaction in cells. Enzymes (and catalysts) take part in a reaction but are unchanged at the end of it. They can be used over and over again but are not used up in the reaction.

Lock and Key Theory

Enzymes have a specific 3D structure. They exactly match the shape of a molecule of substrate.

• Substrate fits into active site of enzyme
• Reaction takes place
• Products leave active site, enzyme ready to work again

Denaturation

If the temperature and pH changes sufficiently beyond an enzyme’s optimum, the shape of the enzyme irreversibly changes. This affects the shape of the active site and means that the enzyme will no longer work. When this happens the enzyme is denatured.

E.g.: an enzyme may look like this:

But if it becomes denatured, it will look more like this:

The substrate will no longer fit in the active site, therefore no products can be created.

Effects of Temperature Substrate Concentration & p H

Factors that affect the rate of a reaction include:

• Temperature
• Enzyme concentration
• Substrate concentration
• Surface area

All enzymes work best at only one particular temperature and pH: this is called the optimum. Different enzymes have different optimum temperatures and pH values.

E.g. This graph suggests that the optimum temperature for this enzyme is around 70 degrees.

Temperature (In degrees)

E.g. This graph suggests that the optimum pH for:

Amylase = 7

These are the conditions in which the enzyme would work best in.

Enzymes will work best if there is a lot of available substrate. When the concentration of the substrate increases, the rate of enzyme activity increases too. However, when an enzyme becomes saturated, no more substrates can fit at any one time even though there is plenty of substrate available. This mean that even when there is a higher substrate concentration, the activity will remain the same as there are not enough enzymes to break down the substrate that is available.

Human Digestive Enzymes

Enzymes work in the digestive system to help us digest food. The break down large indigestible molecules into smaller digestible molecules. They effectively act like scissors cutting up the large molecules. Without enzymes the larger food molecules would not be absorbed into the body through the lining of the small intestine as they cant pass though. The smaller molecules can and so are taken into the blood.

Bile is made in the liver and stored in the gall bladder. It is alkaline to neutralise hydrochloric acid from the stomach. It also emulsifies fat to form small droplets which increases the surface area.

The alkaline conditions and large surface area increase the rate of fat breakdown by lipase.

The “lock-and-key” hypothesis as related to the ionizing radiation effect on genital cuticular structures in the large fruit-tree tortrix Archips podana (Lepidoptera, Tortricidae)

• Published: 30 March 2011
• Volume 91 , pages 7–14, ( 2011 )

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• A. F. Safonkin 1  

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The influence of sublethal doses of ionizing radiation on cuticular structures of the reproductive apparatus of the large fruit-tree tortrix moth was studied. The variability of cuticular teeth of the aedeagus and cuticular projections of the antrum was studied in laboratory cultures after irradiation and in the control. After irradiation of the pupae, numerous protuberances or irregularly arranged small denticles were observed over the entire cuticle of the genitalia. Additional teeth appeared on the aedeagus. The lateral tooth of the aedeagus was more strongly affected by irradiation than the apical one. The influence of irradiation is mostly manifested within the first 24 h of pupa formation. The functional significance of teeth of the aedeagus and projections of the antrum during mating is considered. Based on the variability of aedeagus teeth and external cuticular projections at the antrum inflexion, the applicability of the “lock-and-key” hypothesis to evolution of copulation mechanism in the large fruit-tree tortrix moth is considered.

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Original Russian Text © A.F. Safonkin, 2010, published in Zoologicheskii Zhurnal, 2010, Vol. 89, No. 11, pp. 1331–1339.

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Safonkin, A.F. The “lock-and-key” hypothesis as related to the ionizing radiation effect on genital cuticular structures in the large fruit-tree tortrix Archips podana (Lepidoptera, Tortricidae). Entmol. Rev. 91 , 7–14 (2011). https://doi.org/10.1134/S0013873811010027

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Received : 26 October 2009

Published : 30 March 2011

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DOI : https://doi.org/10.1134/S0013873811010027

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Enzymes & Metabolism ( AQA GCSE Biology: Combined Science )

Revision note.

Biology Lead

Enzymes & metabolism

• Digestive enzymes work outside of cells ; they digest large, insoluble food molecules into smaller, soluble molecules which can be absorbed into the bloodstream
• Metabolism is the sum of all the reactions happening in a cell or organism, in which molecules are synthesised (made) or broken down
• Enzymes speed up chemical reactions in cells, allowing reactions to occur at much faster speeds than they would without enzymes at relatively low temperatures (such as human body temperature)
• Substrates temporarily bind to the active site of an enzyme, which leads to a chemical reaction and the formation of a product(s) which are released
• Some enzymes can process 100s or 1000s of substrates per second

Enzyme specificity diagram

Enzymes are biological catalysts that work in cells, so they randomly move about wherever they are in the cell. They don’t ‘choose’ to collide with a substrate – collisions occur because all molecules are in motion in a liquid

How do enzymes work?

• Enzymes catalyse specific chemical reactions in living organisms – usually one enzyme catalyses one particular reaction:

Enzyme specificity of catalase to hydrogen peroxide diagram

The enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not

• The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
• Proteins are formed from chains of amino acids held together by bonds
• The order of amino acids determines the shape of an enzyme
• If the order is altered, the resulting three-dimensional shape changes

The lock & key model

• The ‘ lock and key theory ’ is one simplified model that is used to explain enzyme action
• The enzyme is like a lock, with the substrate(s) the keys that can fit into the active site of the enzyme with the two being a perfect fit

Diagram showing the lock and key model

• Enzymes and substrates move about randomly  in solution
• When an enzyme and its complementary substrate randomly collide – with the substrate fitting into the active site of the enzyme – an enzyme-substrate complex forms, and the reaction occurs
• A product (or products) forms from the substrate(s) which are then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions

The effect of temperature and pH on enzyme activity

The effect of temperature.

• The specific shape of an enzyme is determined by the amino acids that make the enzyme
• The three-dimensional shape of an enzyme is especially important around the active site area; this ensures that the enzyme’s substrate will fit into the active site enabling the reaction to proceed
• Enzymes work fastest at their ‘ optimum temperature ’ – in the human body, the optimum temperature is around 37°C
• Heating to high temperatures (beyond the optimum) will start to break the bonds that hold the enzyme together – the enzyme will start to distort and lose its shape – this reduces the effectiveness of substrate binding to the active site reducing the activity of the enzyme
• Substrates cannot fit into denatured enzymes as the specific shape of their active site has been lost

Enzyme denaturation diagram

Denaturation is largely irreversible – once enzymes are denatured they cannot regain their proper shape and activity will stop

• Increasing temperature from 0°C to the optimum increases the activity of enzymes as the more energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction
• This means that low temperatures do not denature enzymes, but at lower temperatures with less kinetic energy both enzymes and their substrates collide at a lower rate

The effect of temperature on enzyme activity diagram

This graph shows the effect of temperature on the rate of activity of an enzyme

The effect of pH

• The optimum pH for most enzymes is 7 but some that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) and some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)
• If the pH is too high or too low , the bonds that hold the amino acid chain together to make up the protein can be destroyed
• This will change the shape of the active site , so the substrate can no longer fit into it, reducing the rate of activity
• Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop

If pH is increased or decreased away from the optimum, then the shape of the enzyme is altered

The effect of pH on enzyme activity diagram

This graph shows the effect of pH on the rate of activity of an enzyme from the duodenum

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