Lect 9. Energy & Metabolism - Enzymes
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What is an enzyme?
A catalyst or catalytic protein that speeds up the rate of a reaction without being consumed or altered by the reaction. W/out enzymes, the pathways of metabolism would be too slow to maintain an organism’s biological needs
What is the state in which the reshaping of the starting molecule into a highly unstable state is involved?
the transition state this happens before the reaction can proceed
The energy needed to reshape the reactant molecule is called:
the free energy of activation / activation energy Ea
Ea is often supplied in which form?
It is often supplied in the form of heat that the reactant molecules absorb from their surroundings. This thermal E increases the speed of the molecules, making them collide more and harder causing the bonds to break.
Ea provides a barrier that determines:
the rate at which a reaction will occur.
Heat speeds up a reaction by allowing reactants to reach the transition state more often. But why is using thermal energy inappropriate for biological systems?
• High temp denatures proteins and kills cells • Heat would speed all the cells reactions, not just the necessary ones
Enzymes are very selective in the reactions they catalyze: they determine which chemical reaction will occur, when and where in the cell. What is the location on the enzyme at which the reaction occurs called? (Where does the substrate bind on the enzyme?)
Active Site
Enzyme + substrate(s) =
enzyme-substrate complex (temporary) When enzyme and substrate join, catalytic action of enzyme converts S to product
Most metabolic reactions are reversible: an enzyme can catalyze both _____.
forward and reverse reactions
The capacity of an enzyme to catalyze both reactions depends on:
the concentration of both substrate and products
An active site is typically: describe it
a pocket or groove on the surface of the protein formed by a few AAs
Substrate is held in active site mainly by: bonding?
H-bonds and some ionic bonds
Interaction b/w substrate and the AAs of the protein cause the active site to fit more snuggly around the substrate. This is called:
induced fit It brings chemical groups into position that increase chemical reaction rate. The product leaves the active site and a new substrate enters.
How do enzymes lower the Ea? (4)
• provide template for substrates to come together in proper orientation (otherwise rely on random molecular motion) • can stretch the substrate molecules toward transition state (bending and stressing critical bonds) • can directly participate in the chemical reaction (often involves brief covalent bonding between an active site R-gr and the substrate) • can provide a favourable microenvironment that is more conductive to a particular type of reaction (ex: active site composed of AA.s w/ acidic R-gr creates a pocket of low pH in neutral cell)
The rate at which an enzyme converts a substrate to a product depends on:
initial substrate concentration. The greater the [ ] of S molecules, the more frequently they come in contact w/ enzyme’s active site.
At a certain concentration, all active sites are taken up. This is the substrate concentration at which the enzyme is:
saturated.
At enzyme saturation, the rate of the reaction depends on:
how fast the active site can convert the substrate to product. The cell can speed up the reaction rate by adding more enzyme molecules.
Optimal Enzymatic Conditions Temperature: up to a point, the rate of an enzymatic reaction increases with an increase in temp, because:
there is an increase in collision of substrate molecules with enzyme molecules.
Most enzymes have an optimal pH ranging from
6 to 8
Optimal temperature and pH of an enzyme will vary on type of organism it is found in and where in the organism it is found. ex: lysosomes have a ____ pH
low pH. HCl help denature 4°, 3° and even 2° proteins.
Co-factors Various types of chemicals can serve to promote or inhibit enzyme activity. Prosthetic groups or co-enzymes are:
inorganic or organic molecules that allow the enzyme to function properly. They can be bound tightly to the enzyme or they can bind loosely and reversibly.
A holoenzyme consists of:
• aa (protein) part of the enzyme called apoenzyme • non-protein part called the cofactor
Terminology Summary Simple Protein: made only aa Conjugated Protein is also called:
holoenzyme
The aa part of a holoenzyme is called
the apoenzyme