Embed Size (px)
Citation preview
Cell Biology for Clinical Pharmacy StudentsMD102Module II: Cell Functions (Lecture # 8 )
Dr. Ahmed Sherif [email protected]://sites.google.com/site/ahmedsattia/
Objectives
By the end of this lecture you should be familiar with:
• Metabolic reactions
• Enzymes and how they work
• Factors that affect enzyme activity
• Enzymes classification
• Enzymes inhibitors and regulators
Metabolism
Metabolism is the whole range of biochemical processes that occur within the cell.
Metabolism consists of:
• Breaking down molecules to get energy (catabolism).
• Using the energy to buildup new molecules (anabolism).
Metabolic reactions
How Do Reactions Occur?• Spontaneous reactions may occur very slowly.
• All reactions require free energy of activation (EA).
• Uphill portion represents the EA required to start the reaction.
• Downhill portion represents the loss of free energy by the molecules in the reaction.
. G is the difference in free energy of products and reactants.
All metabolic reaction need energy to start
How can the EA barrier be overcome?
• Increase temperature• Temperatures that are too high denature
organic molecules, so what else is there?
• ENZYMES
• Enzymes lower the EA barrier so that reactions can occur at lower temperatures.
Enzymes
Definition
• Organic (catalysts) in cells that speed up chemical reactions without getting changed or consumed themselves.
• Enzymes are typically proteins, but certain types of RNA can also serve as catalysts. These RNA molecules are called ribozymes.
Enzyme / Substrate Relationship
Substrate:
• It is the reactant upon which an enzyme reacts.
• Enzymes are substrate specific.
• Only the active site of the enzyme actually binds the substrate.
The Active Site
• The reactants and products in enzyme reactions bind to the enzyme at specific active sites.
• The 3-dimensional structures of the reactants and products must be complementary to the 3-dimensional structure of the active site (like hand and glove, or foot and shoe).
The Active Site• Most enzyme-substrate interactions are
the result of weak bonds.
• The active site may cause the enzyme to hold onto the substrate in a very specific way.
• The active site may provide a micro-environment (e.g. low pH) which enhances a reaction.
Induced-Fit Model of enzyme catalysis
• In this model, the enzyme changes shape on substrate binding.
• The active site forms a shape complementary to the substrate only after the substrate has been bound.
• This the currently accepted model.
Enzymes orient substrates and bring them in proximity
Factors that affect enzyme activity
• 1- Temperature:
• Higher temperature generally causes more collisions among the molecules and therefore increases the rate of a reaction.
• More collisions increase the likelihood that substrate will collide with the active site of the enzyme, thus increasing the rate of an enzyme-catalyzed reaction.
Optimum temperature differs between enzymes
Above a certain temperature, activity begins to decline because the enzyme begins to denature.
The rate of the reaction therefore increases with temperature but then decreases.
Factors that affect enzyme activity
2- pH:• Each enzyme has optimal pH that maintains its
normal configuration.
• A change in pH alters ionization of side chains, eventually resulting in denaturation.
Factors that affect enzyme activity
• 3- Enzyme Concentration • If there is insufficient enzyme present, the
reaction will not proceed as fast as it otherwise would because there is not enough enzyme for all of the reactant molecules.
• As the amount of enzyme is increased, the rate of reaction increases.
• If there are more enzyme molecules than are needed, adding additional enzyme will not increase the rate.
• Reaction rate therefore increases as enzyme concentration increases but then it levels off.
• 4- Substrate Concentration:• At lower concentrations, the active sites on most of
the enzyme molecules are not filled because there is not much substrate.
• Higher concentrations cause more collisions between the molecules. With more molecules and collisions, enzymes are more likely to encounter molecules of reactant.
• The maximum velocity of a reaction is reached when the active sites are almost continuously filled.
• Increased substrate concentration after this point will not increase the rate.
• Reaction rate therefore increases as substrate concentration is increased but it levels off.
Cofactors
• Non-protein molecules that help enzymes function.
• Bind to active site to enhance enzymatic reactions.
• Cofactors may be inorganic metals such as zinc, iron, or copper.
• Coenzymes are organic cofactors (e.g. vitamins).
Classification of Enzyme
• Exoenzymes: extracellular; break down large food molecules or harmful agentse.g. Cellulase, amylase, penicillinase.
• Endoenzymes: intracellular enzymes; varied functionse.g. Metabolic enzymes.
• Constitutive enzymes: always present and in relatively constant amounts.
• Induced enzymes: produced only when the substrate is present.
• Enzyme's name is often derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase
• According to the function they can be classified as:
• 1-Oxidoreductases: catalyze oxidation/reduction reactions .
• 2- Transferases: transfer a functional group (e.g. phosphate group).
• 3- Hydrolases: catalyze the hydrolysis of various bonds. • 4- Lyases: cleave various bonds by means other than
hydrolysis and oxidation. • 5- Isomerases: catalyze isomerization changes within a
single molecule.• 6- Ligases: join two molecules with covalent bonds.
Enzyme inhibition
• A molecule with very similar 3-D shape to the substrate may bind to active site, block substrate. This is called a competitive inhibitor.
• Competitive inhibition always depends on relative concentrations of substrate and inhibitor. More inhibitor, more inhibition. Less inhibitor, less inhibition.
• Something that changes protein structure (e.g. by binding to the enzyme at some site outside the active site called the allosteric site) can block enzyme activity. This is called noncompetitive inhibition.
Allosteric Regulation
• Allosterically regulated enzymes have a quaternary protein structure (has 2 or more subunits).
• Each subunit of the enzyme has an active site and an allosteric site.
• Allosteric activators stabilizes the active site and allosteric inhibitors deactivates the active site.
• Cooperativity is the binding of one substrate (or inhibitor) to one active site (or allosteric site) cause the other subunits to assume the same state.
Allosteric Regulation
Feedback inhibition
• Many enzymatic pathways are regulated by feedback inhibition.
• As an enzyme's product accumulates, it turns off the enzyme.
• The end product of the pathway binds to an allosteric site on the first enzyme in the pathway and shuts down the entire sequence.
Summary
