Enzymes

Enzymes are:

• biological catalysts - they speed up chemical reactions without being used up
• they are made of proteins
• they lower the activiation energy of a reaction causing the reaction to take place very quickly at a low temperature and pressueres.

Metabolism is the total of all the chemical reactions an organism needs to survive. Metablism consists of:

• Anabolism - the buildup of substances such as photosynthesis and protein synthesis
• Catabolism - the breakdown of substances such as during respiration and digestion

Substrates of a reaction are converted to Products

• Enzymes are globular proteins with a complex 3D shape
• They have a depression on thier surface called the ACTIVE SITE into which subrates fit
• Enzymes are specific becuase thier active sites has a complementary shape to one substrate or set of substances - therefore, they do not work on other molecules
• If the substate collides successfully with the active site, it will bind to the active site forming an enzyme subrate complex
• This brings the reactants together and stresses covalent bonds allowing the reaction to occur at lower temeratures and pressures
• The products formed are released from the enzyme
• Many reactions need an imput of engery to start the reaction off. This is energy called the activation engery
• Enzymes are catalysts. They lower the activation energy needed for the reaction to take place. This increases the rate of reaction

The active site is ridgid and does not change shape:

• The substances entres an active site that is complementry to the substrate
• Bonds in the substrate break converting the substrate to product

The active site is flexible and changes shape when the substrate binds:

• The active site is not prescise fi to the substrate
• The substrate entres the active site
• The substrate binds to the active site with weak bonds causing the active site to change shape so that it is complementry to the substrate
• Bonds in the substrate breka converting the substrate to product
• The products leave the active site and it reverts to the orginal shape

A metabollic pathway is a series of chemical reactions occuring within a cell. In a pathway, the initial chemical is modified by a sequence of chemical reactions. These reactions are catalysed by enymes, where the product of one enzyme acts as the substrate for the next.

Some enzymes work inside the cells that made them. A wide range of enzymes are needed for cell metabolism, for example, enzymes are needed for digestion of old organelles, synthesis of proteins and ATP and conversion of toxins to less harmful molecules.

Some enzymes are released outside of cells. Some of these only become functional in the conditions outside the cell, for example, pepsin becomes active when released into the acid condition in the stomach where it breaks down proteins to amino acids. Some extracellular enzymes are released outisde the body, for example, flies release enzymes onto thier food and **** up the partially digested material.

Starch is made of many glucose molecules joined by glycosidic bonds.

Salivary amylase is released into the mouth and pancreatic amylase is released into the duodenum (small intestine).

Amylase hydrolyses the glycosidic bonds in the starch forming maltose.

Maltose is hydrolysed by the enzyme maltase forming glucose, a small soluble molecule that can be absorbed into the blood.

Glycosidic bonds in the carbohydrate are broken by hydrolysis which involves the addition of water to break the bond.

Protease enzymes such as pepsin (found in the stomach) and trypsin (in the small intestine) digest proteins into smaller polypeptides. Other peptidases hydrolse the bonds in these polypeptides producing amino acids that are small and soluble and can be absorbed into the blood.

Enzymes have an optimum temperature at which they work fastest.

As temperatures increase up to the optimum, ezyme activity increases becuase:

• increased jinetic energy causes enzymes and substrates to move faster
• Therefore more successful collisions between enzyme and substrate
• more enzyme-substrate complexes are formed

The rate of reaction doubles with every 10 degree rise in temperature. Temperature coefficient Q10=2.

Above the optimum:

• the increased kineti energy causes the hydrogen bonds holding the tertiary structure to break
• this causes the enzyme to change shape so the active site is no longer complementary to the substrate. This is called denaturation
• Less enzyme-substrate complexes form and the reaction slows down as a result

Enzymes have an optimum pH at which they work fastest. Buffers can be used to keep the pH of a solution constant when investigating the effect of different pH on enzyme activity.

pH is a mesure of the concentration of hydrogen ions in solition. Acidic solutions have a high concentration of hydrgen ions and a low pH.

The active site of an enzyme is only the right shape to fit with its substrate at the optimum pH. If the pH changes above or below the optimum the change in hydrogen ion concentration changes the chare distribution on the enzyme resulting in:

• the hydrogen and ionic bonds that hold the tertiary structure of the enzyme in shape break
• the enzyme denatures
• the shape and charges of the active site change so the substrate is no longer able to bind to the active site, slowing the rate of reaction. Less E-S complexes are formed
• extreme changes in pH can result in a permanent change in structure

For an enzyme to catalyse a reaction, the enzyme and substrate need to collide successfully to form an enzymes-substrate complex.

Increasing Substrate Concentration: if the substrate concentration is very low (a dilute solution), the enzymes will collide with substrate infrequently and the reaction rate will be slow. Substrate concentration is a limiting factor. When more substrates are added to the solution, they collide with enzymes more frequently forming enzyme-substrate complexes at a faster rate.

if substrate concentration is increased further, a maximum rate of reaction will be reached where the enzymes are working at thier maximum rate as all the nezymes active sites are occupied. Increasing substrate concentration further will have no effect on the rate of reaction.

If the substrate is a solid, its surface area also has an impact on the rate of reaction. The larger the surface area, the faster the rate of reaction.

Incresing enzyme-substrate concentration also increases the rate of reaction becuase there are more collisons meaning there are more enzyme-substrate complexes.

• competitive inhibitors are similar in shape to the substrate
• they can bind to the active site preventing the substrate from entering so less E-S complexes form, slowing the rate of production formation
• the inhibitor and substrate 'compete' for the active site
• as substrate concentration is increased competitive inhibiton is almost overcome
• most only bind temporarily to the enzyme- these are reverible inhibitors

Example: statins and aspirin

• non-competitve inhibitors bind to an allosteric site (not the active site)
• the binding of the inhibitor changes the shape of the enzyme so the active site is no longer complememntry to the substrate
• the substrate cannot entre the active site so less E-S complexes form, slowing the rate of product formation
• increasing the concentration of substrate does not overcome the effect of non-competitive inhibitor
• increasing the concentration of inhibitor decreases the rate of reaction further
• some are reversible inhibitors and some are irreversible ad cannot be removed from the enzyme once they attach

Examples: organophophate and proton pump inhibitors

To ensure that resources are not wasted by production of too much of a product, the enzymes of many metbolic pathways are inhibited by their product. This is a negative feedback control mechanism. Once thr product has been used, the inhibition is removed and the synthesis of the product continues.

Example: In repsiration, when levels of ATP are high, ATP inhibits phosphofructokinase, the first of several enzymes involved in the conversion of glucose to CO2, H2O, and ATP.

Some enzymes need a non-protein group to bind to them beofre they can function. These groups may form part of the active site of the enzyme or they may help transfer atoms or groups from one reaction to another in a multi-step pathway.These molecules are called cofactors. If the cofactor is an organic molecule it is called a coenzyme.

An enzyme with no cofactor attached is called a APOENZYME and when a cofacotr bind to the enzyme it is called HALOENZYME.

Prosthetic Group: are similar to cofactos but they are tightly bound and are permanent feature of the protein. Example: Zinc ions form part of the structure of carbonic anhydrase.

Precursor Activiation: Many enzymes, especially ones that could damage the cell, are made in an inactive form. These precursor enzymes can be activated by:

• a cofactor
• another enzyme
• change in pH or temperature