Definition
A protein produced by a living organism which acts as a catalyst to bring about a specific biochemical reaction.
Most enzymes are proteins with large complex molecules whose action depends on their particular molecular shape (we now know that this is their tertiary structure). Some enzymes control reactions within cells (intra-cellular) and some, such as the enzymes involved in digestion, outside them (extra cellular)
Most enzymes are proteins with large complex molecules whose action depends on their particular molecular shape (we now know that this is their tertiary structure). Some enzymes control reactions within cells (intra-cellular) and some, such as the enzymes involved in digestion, outside them (extra cellular)
Lock and Key Theory and Induced Fit Theory
At GCSE level you learned about the 'lock and key' theory. Essentially this still stands, but, at A-level you need to know that we now believe that the enzyme is able to change shape a little ignored to fit the substrate. This id referred to as the 'induced fit model or theory'.
Take a look at these:
Take a look at these:
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Lysosyme
Lysozyme: a model of enzyme actionA number of lysozymes are found in nature; in human tears and egg white, for examples. The enzyme is antibacterial because it degrades the polysaccharide that is found in the cell walls of many bacteria. It does this by catalyzing the insertion of a water molecule at the position indicated by the red arrow (a glycosidic bond). This hydrolysis breaks the chain at that point.
Link to discussion of the bacterial
cell wall and how it is affected
by certain antibiotics.Link to view of the primary structure of
hen's egg white lysozyme.
The bacterial polysaccharide consists of long chains of alternating amino sugars:
Lysozyme is a globular protein with a deep cleft across part of its surface. Six hexoses of the substrate fit into this cleft.
As for lysozyme itself, binding of the substrate induces a small (~0.75Å) movement of certain amino acid residues so the cleft closes slightly over its substrate. So the "lock" as well as the "key" changes shape as the two are brought together. (This is sometimes called "induced fit".)
The amino acid residues in the vicinity of rings 4 and 5 provide a plausible mechanism for completing the catalytic act. Residue 35, glutamic acid (Glu-35), is about 3Å from the -O- bridge that is to be broken. The free carboxyl group of glutamic acid is a hydrogen ion donor and available to transfer H+ to the oxygen atom. This would break the already-strained bond between the oxygen atom and the carbon atom of ring 4.
Now having lost an electron, the carbon atom acquires a positive charge. Ionized carbon is normally very unstable, but the attraction of the negatively-charged carboxyl ion of Asp-52 could stabilize it long enough for an -OH ion (from a spontaneously dissociated water molecule) to unite with the carbon. Even at pH 7, water spontaneously dissociates to produce H+ and OH- ions. [Discussion] The hydrogen ion (H+) left over can replace that lost by Glu-35.
In the 20 August 2001 issue of Nature, Vocadlo, D. J., et al., report evidence that Asp-52 stabilizes ring 4 by forming a transient covalent bond rather than through ionic interactions.In either case, the chain is broken, the two fragments separate from the enzyme, and the enzyme is free to attach to a new location on the bacterial cell wall and continue its work of digesting it.
Link to discussion of the bacterial
cell wall and how it is affected
by certain antibiotics.Link to view of the primary structure of
hen's egg white lysozyme.
The bacterial polysaccharide consists of long chains of alternating amino sugars:
- N-acetylglucosamine (NAG)
- N-acetylmuramic acid (NAM)
Lysozyme is a globular protein with a deep cleft across part of its surface. Six hexoses of the substrate fit into this cleft.
- With so many oxygen atoms in sugars, as many as 14 hydrogen bonds form between the six amino sugars and certain amino acid R groups such as Arg-114, Asn-37, Asn-44, Trp-62, Trp-63, and Asp-101.
- Some hydrogen bonds also form with the C=O groups of several peptide bonds.
- In addition, hydrophobic interactions may help hold the substrate in position.
As for lysozyme itself, binding of the substrate induces a small (~0.75Å) movement of certain amino acid residues so the cleft closes slightly over its substrate. So the "lock" as well as the "key" changes shape as the two are brought together. (This is sometimes called "induced fit".)
The amino acid residues in the vicinity of rings 4 and 5 provide a plausible mechanism for completing the catalytic act. Residue 35, glutamic acid (Glu-35), is about 3Å from the -O- bridge that is to be broken. The free carboxyl group of glutamic acid is a hydrogen ion donor and available to transfer H+ to the oxygen atom. This would break the already-strained bond between the oxygen atom and the carbon atom of ring 4.
Now having lost an electron, the carbon atom acquires a positive charge. Ionized carbon is normally very unstable, but the attraction of the negatively-charged carboxyl ion of Asp-52 could stabilize it long enough for an -OH ion (from a spontaneously dissociated water molecule) to unite with the carbon. Even at pH 7, water spontaneously dissociates to produce H+ and OH- ions. [Discussion] The hydrogen ion (H+) left over can replace that lost by Glu-35.
In the 20 August 2001 issue of Nature, Vocadlo, D. J., et al., report evidence that Asp-52 stabilizes ring 4 by forming a transient covalent bond rather than through ionic interactions.In either case, the chain is broken, the two fragments separate from the enzyme, and the enzyme is free to attach to a new location on the bacterial cell wall and continue its work of digesting it.
Activation energy
All chemicals have a certain amount of energy, it is stored in bonds when they form. Different molecules have different amounts of energy depending on the number and type of bonds present.
In general, substances will react in such a way as to end up with a level energy (less chemical energy). With less energy they are said to be more stable. However, it rarely happens spontaneously. Usually some energy must be put in first to start the reaction. This is called the activation energy. With the match we apply heat (usually from friction), the heat energy is enough to break some of the bonds in the reactant causing a reaction to take place releasing the chemical energy as heat. Once the activation energy has been over come the reaction will continue until the it is over and the whole substance all has the lower amount of stored energy.
Catalysts, including enzymes, reduce activation energy causing reactions to take place at lower temperatures (i.e. with less energy input). Many of the chemical reactions in living thinks (metabolic reactions) would not take place without enzymes.
A match is a nice analogy. An unlit match has lots of stored energy when compared to a match that has burned out.
In general, substances will react in such a way as to end up with a level energy (less chemical energy). With less energy they are said to be more stable. However, it rarely happens spontaneously. Usually some energy must be put in first to start the reaction. This is called the activation energy. With the match we apply heat (usually from friction), the heat energy is enough to break some of the bonds in the reactant causing a reaction to take place releasing the chemical energy as heat. Once the activation energy has been over come the reaction will continue until the it is over and the whole substance all has the lower amount of stored energy.
Catalysts, including enzymes, reduce activation energy causing reactions to take place at lower temperatures (i.e. with less energy input). Many of the chemical reactions in living thinks (metabolic reactions) would not take place without enzymes.
A match is a nice analogy. An unlit match has lots of stored energy when compared to a match that has burned out.