Case 2 BIOCHEM - enzymes, micronutrients Flashcards
Define an enzyme
- proteins that catalyse biochemical reactions
Enzymes’ role in digestion
- digest food by hydrolysing bonds that link the building blocks of dietary macromolecules
Function of digestive enzymes
- hydrolyse peptide bonds in dietary proteins. Breakdown of proteins into peptides and then into amino acids for absorption
- hydrolyse glycosidic bonds in dietary carbohydrates. Breakdown of complex carbohydrates into monosaccharides and disaccharides for absorption
- hydrolyse ester bonds in dietary fats. Breakdown of fat into fatty acids, glycerol, and monoacyglycerol for absorption
Enzyme specificity
- type of reaction (oxidation, hydrolysis or isomerisation)
- nature of substrates
- defined by the active site structure
Active site
Small area of an enzyme where substrates bind and reaction takes place
Roles of amino acid residues at the active site
- bind substrates in the correct orientation for the reaction to occur
- take part in catalysis of the reaction
Carboxypeptidase
- hydrolyse peptide bond at the amino acid residue at the C terminus
Pancreatic Serine Proteases
- chymotrypsin, elastase and trypsin use the same mechanism for catalysis but have different active sites that have different substrate specificities
- chymotrypsin: hydrolyses the peptide bond at the bulky hydrophobic amino acid residue
- elastse: hydrolyses peptide bond at a small amino acid residue
- trypsin: hydrolyses peptide bond at a Lys or Arg (basic) amino acid residue
Thermodynamically favourable reactions
free energy change (delta G) is negative
Thermodynamically speaking, how does an enzyme catalyse a reaction?
- by lowering the activation energy. This increases the rate and the fraction of molecules reaching the transition stage.
- the lowest the activation energy, the higher the fraction of substrate molecules which attain the transitions state thus the higher the reaction rate.
Mechanisms of enzyme catalysis (molecular)
- straining substrate conformation so it starts to resemble the products
- covalently binding substrate in order to activate the substrate
- providing a suitable environment at the active site for the reaction to occur
- bringing substrates in close proximity and in correct orientation for reaction to occur at the active site
- acting as a base or acid
* different enzymes use different combinations of the mechanisms
Shapes of enzyme-catalysed reactions
- Michaelis-Menten = hyperbolic.
2. Allosteric = sigmoid.
Define Km
substrate concentration at which v=1/2vmax
Significance of Km in the context of enzyme sensitivity
- [s] «_space;km: enzyme will be sensitive to changes in [s]
- [s] = km: enzyme will be half saturated with substrate
- [s]» km: enzyme will be insensitive to changes in [s] and will be operating at Vmax
Significance of Km in the context of enzyme sensitivity
- [s] «_space;km: enzyme will be sensitive to changes in [s]
- [s] = km: enzyme will be half saturated with substrate
- [s]» km: enzyme will be insensitive to changes in [s] and will be operating at Vmax
Michaelis-Menten equation
v = Vmax[s]/Km+[s]
Define enzyme inhibition
- small molecules that decrease the rate of enzyme-catalysed reactions
Differentiate between reversible and irreversible inhibitors
- reversible: binds non-covalently
- irreversible: binds covalently
Define competitive inhibition
- inhibitors binding at the active site where the substrate binds.
- this prevents the binding of substrate to the enzyme
- binding of I or S to E are mutually exclusive (i.e. EIS complex cannot form)
Effect of competitive inhibition on Km
Vmax is attained at a higher [s] as inhibitors have to be competed out. Therefore Km increases.
How is the pancreas protected from destruction by proteases
- pancreatic trypsin inhibitor
2. synthesis of pancreatic proteases as inactive zymogens
Define positive cooperativity
binding of substrate at one active site increases the affnity for substrate binding at vacant active sites on remaining subunits.
How do the effectors of allosteric enzymes influence its affinity for substrate binding
the binding of substrate at the active site is regulated by the binding of an effector at a regulatory site
How do the effectors of allosteric enzymes influence its affinity for substrate binding
the binding of substrate at the active site is regulated by the binding of an effector at a regulatory site
- positive effectors activate the enzyme, thus shifting the sigmoid curve to the left
- negative effectors inhibit the enzyme, shifting the sigmoid curve to the right
