enzymes Flashcards
What are enzymes?
-Proteins that act as biological catalysts
-Sped up the rates of chemical reactions
-Do not alter the position of the chemical equilibrium governing the substrate(s) and product(s). Enzymes simply facilitate a quicker route from substrate to product
-Enzymes (catalysts) are not consumed during the chemical reaction. They are always regenerated at the end
-Enzymes are often subjected to regulation to control their activity
-Enz + Substrate -> Enz + Product
How do enzymes provide chemical specificity?
-Substrate: a reactant for an enzyme
-Most enzymes will only work on one or a very few substrates (i.e. one distinct chemical compound)
-Enzymes generally only yield one product from a given substrate
-Many enzymes are stereospecific. They will typically only catalyze the conversion of a particular steroisomer or configuration of a molecule (e.g. only a D sugar vs. the L sugar)
What is the active site of an enzyme?
-Where substrate binding and the chemical reaction take place
-R-groups from the enzyme polypeptide chain making up the active site can be dispersed in the primary sequence
-Active site generally takes up a small amount of proteins entire surface area (needs rest of protein to provide structure)
Describe what happens at the active site
- Substrate(s) binds. The R-groups and peptide backbone atoms at the enzyme active site recognize and interact with the substrate, making favourable interactions
- Chemistry occurs. Amino acid R-groups perform chemistry (usually by adding or removing a proton) and also stabilize any high-energy reaction intermediates
- Products released
Explain substrate binding by enzymes
-Enzymes bind their substrate in a single specific orientation that favors catlysis (conversion to product)
-Substrate binding occurs via weak non-covalent interactions
1. van der Waals forcers favored by complementary surface shape of the substrate and the enzyme active site residues
2. Polar groups of substrate make hydrogen bonds with polar R-groups at the active site
3. If the substrate carries and electric charge (eg. carboxylate, amino or phosphate group), there are often oppositely charged R-groups from the enzyme acitve site that then make salt bridges to the substrate
What are the two models for substrate binding?
Lock and key:
-substrate fits perfectly into enzyme active site much like a lock fits into a key
Induced fit:
-Small conformational changes (eg. reorientation of one or more side chains) at the active site of the enzyme accomodate substrate binding
What is the conformational change of hexokinase upon binding its substrate of glucose?
-Composed of two domains
-Induced fit
-When glucose and ATP bind at the active site, the two domains move closer together
What is the coenzyme of Monoamine oxidase?
Flavin adenine dinucleotide (FAD)
What is the coenzyme of lactate dehydrogenase?
Nicotinamide adenine dinucleotide (NAD+)
What is the coenzyme of Acetyl CoA carboxylase?
Coenzyme A (CoA)
How can enzymes be classified?
-Based on the type of chemical conversion they catalyze
-Mostly act on small metabolites (sugars, fats, nucleic acids, amino acids) in the breakdown of nutrients and in the synthesis of cellular precursors
-However, enzymes can also act on much larger substrates such as proteins or nucleic acids
Classify the following enzymes: proteinases, kinases and transferases
Proteinases: Hydrolyze peptide bonds in other protein substrates
Kinases: transfer a phosphoryl group (usually from ATP) to an acceptor molecule (often a small metabolite; must contain an acceptor OH group; sometimes another protein)
Transferases: Move a functional group from one molecule to another (eg. an acetyl group, an acyl group, a glucosyl group)
What are some special enzymes that convert different forms of energy?
-Energy of ATP hydrolysis is converted into mechanical movement (muscle contraction)
-Light energy converted into energy of chemical bonds (photosynthesis)
-Energy of cellular transmembrane proton gradient converted into energy of covalent bond formation (F0F1 ATP synthase)
-Energy of ATP hydrolysis is often used to acctivate metabolites for biosynthetic reactions
Describe the catalytic enhancement of reaction rates as a result of enzymes
-The rate acceleration of 10^3 to 10^7 over the uncatalyzed process
-Biological conditions: the reaction proceeds in aqueous solution at ambient temp and mild conditions (ie. neutral pH, 25C)
-Most cellular chemical reactions do not occur at a perceptible rate in the absence of an enzyme
How do some enzymes act on other proteins and alther their chemical structure?
-Some enzymes act on other proteins (the other protein is the substrate) and modify the other proteins’ chemical structure and biological properties
-Addition of a phosphate group to a suitable R-group on another protein’s primary sequence (usually Ser, Thr, or Tyr OH group; protein kinase)
-Cleaving another protein’s polypeptide chain in two by breaking a specific peptide bond and adding water (protein hydrolase or proteinase)
-Addition of sugar moiety to another protein (glycosyl transferase)
