• Enzyme Catalysts vs. Chemical Catalysts:
  1. higher reaction rates 10^6 to 10^12 greater than corresponding uncatalysed reactions
  2. Milder reaction conditions: Extremes of pH, T and P not required.
  3. Greater reaction specificity: Rarely have side products.
  4. Capacity for regulation.

1. Historical Perspective of Enzymology

• 1810 Joseph Gay-Lussac determines that ethanol and CO2 are products of sugar decomposition by yeast.
• 1835 Jacob Berzelius observes that diastase catalyzes the hydrolysis of starch better than sulphuric acid.
• 1984 Emil Fisher discovers enzymes distinguish stereoisomeric sugars and formulates lock-and-key hypothesis.
• 1926 James Sumner crystallizes jack bean urease which catalyses urea to NH3 and CO2.
• 1963 structure of bovine pancreatic ribonuclease A is reported
• 1965 hen egg white lysozyme structure determined by X-ray

2. Substrate Specificity

• The noncovalent forces through which substrates and other molecules bind to enzymes are identical in character to the forces that dictate the conformations of the proteins themselves. Both involve van der Waals, electrostatic, hydrogen bonding, and hydrophobic interactions.
• X-Ray studies indicate that the substrate-binding sites of most enzymes are largely preformed but that most of them exhibit at least some degree of induced fit upon binding substrate.
• Note to self: This suggests a parametric scale of 0.0 to 1.0, with 0.0 being lock and key and 1.0 being induced fit, being a useful way to characterize the position that enzymes occupy in the spectrum between these two models.

• A. Stereospecificity
  

• Enzymes are highly specific both in binding chiral substrates and in catalyzing their reactions by virtue of their inherent chirality. Proteins consist only of L-amino acids.

  Reaction:

  Ethanol

  +

  NAD+

  YADH

  Acetaldehyde

  + H+  +

  NADH

• Ethanol's two methylene H atoms may be distinguished if the molecule is held in some sort of asymmetric jig. The substrate-binding sites are, of course, just such jigs since they immobilize the reacting groups of the substrate on the enzyme surface.
• Enzymes are absolutely stereospecific in the reactions they catalyze.
• Substrate-binding sites of enzymes are jigs that immobilize the reacting groups of the substrate on the enzyme surface.
• The stereospecificity of YADH is not unusual. Nearly all enzymes that participate in chiral reactions are absolutely stereo specific.

  B. Geometric "chemical" specificity

• Protein + Cofactor = holoenzyme
• Protein - Cofactor = apoenzyme

3. Coenzymes

• Nicotinamide is a component of NAD+. Niacin is its carboxylic analog. It relieves the Vitamin B-3 deficiency known as pellagra. Soaking corn in lime catalyzes the niacinamide to niacin conversion, thus increasing nutritional value.
  

  Nicotinamide (niacinamide)	Nicotinic Acid (niacin)

• The enzyme sans cofactor is called an apoenzyme.
• Many vitamins are coenzyme precursors.
• The vitamins in the human diet that are conenzyme precursors are all water soluble. Lipid soluble vitamins such as A and D are not components of coenzymes.

4. Regulation of Enzymatic Activity

• Aspartate transcarbamoylasen (ATCase) catalyzes the formation of N-carbamoylaspartate from carbamoyl phosphate and aspartate:

Carbamoyl Phosphate

+

Aspartate

|

Aspartate Transcarbamoylase

N-Carbamoyl Aspartate

+

Phosphoric Acid

• ATCase is inhibited by cytidine triphosphate (CTP) and activated by adenosite triphosphate (ATP).
• The Feedback Inhibition of ATCase Regulates Pyrimidine Biosynthesis.
• Feedback inhibition is a common mode of metabolic regulation in which the concentration of a biosynthetic pathway product controls the activity of an enzyme near the beginning of that pathway.
• Allosteric Changes Alter ATCase's Substrate-Binding Sites
• ATCase's tertiary and quaternary shifts are so tightly coupled through extensive intersubunit contacts that they cannot occur independently. This is similar to Hemoglobin.
• Allosteric Transitions in Other Enzymes Resemble Those of Hemoglobin and ATCase.

5. Enzyme Nomenclature

Problem Set