Enzymes

Demonstrate an understanding of the following terms: metabolism, enzyme, substrate, coenzyme, activation energy

[A chemical reaction involves bond breaking and bond forming. When a reaction rearranges the atoms of molecules, existing bonds in the reactants must be broken and the new bonds of the products must be formed. These processes require exchanges of energy between the mixture of molecules and the surrounding environment.]

Metabolism - is the sum of all chemical reactions occuring inside a living cell. Reactions in cells are usually part of a metabolic pathway.

Metabolic pathways begin with a particular reactant and terminate with an end product with many small steps in between. In the pathway one reaction leads to the next reaction in an organized, highly structured manner. This arrangment makes it possible for one pathway to lead to several others. Also, metabolic energy is captured and utilized more easily if it is released in small increments [rather than all at once]. The enzymes in a series can be located adjacent to each other (in an organelle or in the membrane of an organelle), thus speeding the reaction process. Also, intermediate products tend not to accumulate, making the process more efficient. By removing intermediates (and by inference end products) from the reactive pathway, equilibrium (the tendency of reactions to reverse when concentrations of the products build up to a certain level) effects are minimized, since equilibrium is not attained, and so the reactions will proceed in the "preferred" direction.

See diagram bottom page 75 and read last paragraph

Enzyme - Enzymes allow many chemical reactions to occur within the homeostasis constraints of a living system. Enzymes function as organic catalysts. A catalyst is a chemical involved in, but not changed by, a chemical reaction. Many enzymes function by lowering the activation energy of reactions. By bringing the reactants closer together, chemical bonds may be weakened and reactions will proceed faster than without the catalyst. Every reaction in a cell requires a specific enzyme. In most instances, an enzyme is a protein molecule that functions as an organic catalyst to speed up chemical reactions. [It was previously believed that all enzymes are proteins until the recent discovery of catalytic RNA molecules called ribozymes] [In the cell, an enzyme brings together particular molecules and causes them to react with one another just like a mutual friend might bring together two people at a crowded school dance] An enzyme is very specific in its action and can speed up only one particular reaction. Ex. Sucrase speeds up the breakdown of Sucrose (+H2O) into its component molecules Glucose and Fructose. [Enzymes are named for their substrates (see table 5.1) or for the action they perform (dehydrogenase removes hydrogen atoms from its substrate] An enzymes specificity is tied to its unique shape. Enzymes can act rapidly, as in the case of carbonic anhydrase (enzymes typically end in the -ase suffix), which causes the chemicals to react 107 times faster than without the enzyme present. Carbonic anhydrase speeds up the transfer of carbon dioxide from cells to the blood. There are over 2000 known enzymes, each of which is involved with one specific chemical reaction. Enzymes are substrate specific. The enzyme peptidase (which breaks peptide bonds in proteins) will not work on starch (which is broken down by human-produced amylase in the mouth).

Substrate - [A reactant is a substance that participates in a reaction] the reactants in an enzymatic reaction are called the substrates for that enzyme.

Coenzyme - Many enzymes require a nonprotein cofactor to assist them in carrying out their function. [Some are ions such as Mg, K or Ca.] Coenzymes are cofactors that consist of organic molecules that bind to enzymes and serve as carriers for chemical groups or electrons. The protein portion of the enzyme accounts for its specificity and the coenzyme portion of the enzyme participates in the reaction. A coenzyme is generally a large molecule that the body is incapable of synthesizing without the ingestion of a vitamin.

Activation energy (Ea) - The initial investment of energy for starting a reaction - the energy required to break the bonds of the reactant molecules - is known as the activation energy. [It acts as a barrier that keeps the molecules of life from spontaneously decomposing. Occasionally however, cells need for reactions to occur.]

Enzymes are absolutely necessary to the continued existence of a cell because they allow reactions to occur at moderate temperatures. Enzymes lower the activation energy - the amount of heat needed for a reaction to occur. The hydrolysis of casein , the protein found in milk requires 20, 600 Kcal/gram molecular weight to occur when there is no enzyme and only 12,000 Kcal with the enzyme. Enzymes lower the activation energy by binding with their substrates in such a way that a reaction can occur more readily.

Identify the source gland for thyroxin and relate the function of thyroxin to metabolism

The source gland for thyroxin is the thyroid gland located in the neck attached to the trachea just below the laryrnx. Thyroxin stimulates most cells of the body to metabolize at a faster rate. The number of respiratory enzymes in the cell increases as does oxygen uptake.

[Read Page 352 up to calcitonin and observe figures 19.9- 19.12.]

Explain the "lock and key" model of enzymatic action

Enzymes are proteins. The functioning of the enzyme is determined by the shape of the protein. The arrangement of molecules on the enzyme produces an area known as the active site within which the specific substrate(s) will "fit". It recognizes, confines and orients the substrate in a particular direction. Enzymes form a complex a complex with their substrates. [Study figure 5.4 page 77] The enzyme does not undergoe a permanent change so it can be used over and over again. Therefore only a small amount of enzyme actually is found in a cell. The shape of the enzyme allows an enzyme-substrate complex to form which explains the specificity of an enzyme. The substrates are seemingly specific to the enzyme because their shapes fit together as a key fits a lock. [They are held in place by weak interactions such as hydrogen bonds and ionic bonds] The induced fit hypothesis (shown above) suggests that the binding of the substrate to the enzyme alters the structure of the enzyme, placing some strain on the substrate and further facilitating the reaction.

Identify the role of vitamins in biochemical reactions

A coenzyme is generally a large molecule that the body is incapable of synthesizing without the ingestion of a vitamin. Vitamins are organic dietary requirments needed in small amounts only. Niacin, thiamin (vitamin B1) riboflavin, folate, and biotin are just a few examples of well known vitamins that are parts of coenzymes.

 

Differentiate between the roles of enzymes and co-enzymes in biochemical reactions

An enzyme is a protein molecule that functions as an organic catalyst to speed up chemical reactions. Coenzymes are cofactors that consist of organic molecules that bind to enzymes and serve as carriers for chemical groups or electrons. The protein portion of the enzyme accounts for its specificity and the coenzyme portion of the enzyme participates in the reaction. Coenzymes are nonprotein organic molecules bound to enzymes near the active site. NAD (nicotinamide adenine dinucleotide) is an example.

 

Apply knowledge of proteins to explain the effects on enzyme activity of pH, temperature, substrate concentration, enzyme concentration, competitive inhibitors, and heavy metals

[Recall from our discussion of biological molecules that the three-dimensional structure of enzymes and other proteins are sensitive to their environment.]

Each enzyme has optimal conditions in which it works best, because that environment favors the most active conformation of the enzyme

Optimum temperature results in more product (figure 5.5)

Up to a certain point, the velocity of an enzymatic reaction increases with increasing temperature, partly because substrates collide with active sites more frequently when the molecules move more rapidly. But at some point on the temperature scale, the speed of the enzymatic reaction drops sharply with additional increases in temperature. The thermal agitation of the enzyme molecule disrupts the hydrogen bonds, ionic bonds, and other weak interactions that stabilize the active conformation, and the protein molecule denatures. Each enzyme has an optimum temperature at which its reaction rate is fastest. This temperature allows the greatest number of molecular collisions without denaturing the enzyme. [Most human enzymes have optimal temperatures of 35 to 40 degrees Celcius. Bacteria that live in hot springs contain enzymes with optimal temperatures of 70 degrees or higher.]

Optimum pH results in more product

The optimal pH range for most enzymes is 6 to 8 but there are exceptions. Pepsin, a digestive enzyme in the stomach, works best at pH 2. Trypsin, a digestive enzyme residing in the alkaline environment of the intestine, has an optimal pH of 8.

 

 

 

 

 

 

 

Competitive inhibition results in less product

A compettitive inhibitor mimics the substrate and competes for the active site blocking the substrate from entering the active site thus resulting in less product.

 

 

 

 

 

Noncompetitive Inhibition occurs when the inhibitory chemical, which does not have to resemble the substrate, binds to the enzyme other than at the active site. Lead binds to SH groups in this fashion. Irreversible Inhibition occurs when the chemical either permanently binds to or massively denatures the enzyme so that the tertiary structure cannot be restored. Nerve gas permanently blocks pathways involved in nerve message transmission, resulting in death. Penicillin, the first of the "wonder drug" antibiotics, permanently blocks the pathways certain]

Adding substrate results in more product

Assuming there are enough enzymes with available active sites more substrate will result in more product. Increasing the concentration of product can also help to overcome the effects of competitive inhibition because substrate will be more likely to bind than the inhibitor as sites become available.

Adding enzyme results in more product

Assuming there are enough substrate molecules adding more enzymes with available active sites will result in more product.

Devise an experiment using the scientific method

Be able to describe an experiment you would conduct to test the effects of some substance on enzyme activity.

 

 

 

 

 

 

The diagrams illustrate a reaction that occurs in the small intestine. Give the specific name for each of the following.

a) Molecule X:

 

b) Molecule Y:

 

b) In a laboratory experiment, substance Y was added in increasing amounts until it eventually had no effect on the rate of the reaction. Explain why.

 

 

c) A solution containing lead ions was added to the reaction. How will the addition of this

solution affect the reaction? Explain why.

 

 

 

You could be asked to draw a labelled diagram to illustrate the "lock and key" model of enzymatic action. If this happens draw and label the following as your answer.