Enzyme Activity

Enzyme Kinetics

Before we can fully discuss enzyme kinetics, please look at the figure below to understand the rate reaction where k = rate constant. Enzyme catalysis is divided into:

• The first rate for the enzyme binding to the substrate
• The second rate for the substrate forming the product, plus the enzyme (since enzymes are NOT consumed in the reaction).

• Rate is the speed at which the reaction proceeds. (Written as V)
• When there is very high amounts of substrate, the enzyme will be saturated and the reaction will proceed at a maximum capacity, known as V_max

The following figure depicts a saturation curve:

Michaelis–Menten Kinetics

• Steady State: is a point where the enzyme-substrate complex concentration is constant. Essentially, the formation of the of the ES complex is equivalent to the dissociation of the complex. Remember: it is not necessary that the enzyme-substrate complex forms the products, the reverse reaction can allow them to disassociate back into individual enzyme and substrate molecules.
• Michaelis-Menten Equation: (you must memorize this)
  

• Vo = Reaction Rate
• V_max = Maximum reaction rate possible
• [S] = Concentration of substrate
• Km = Substrate concentration at 1/2 V_max

Cooperativity

Not all enzymes follow the kinetic pattern above such as in the cases of enzymes that bind more than one substrate molecule. Cooperative enzymes have more than one active site.

• Positive Cooperativity: Binding of one substrate molecule increases the affinity for another molecule to bind (most common)
• Negative Cooperativity: Binding of one substrate molecule decreases the affinity for another molecule to bind (less common)

We will ignore any further detail about negative cooperativity. The resulting curve from positive cooperativity is sigmoidal:

Note: This cooperative binding phenomenon is not exclusive to enzymes, for example: Hemoglobin shows this same cooperative binding phenomenon with oxygen.

Feedback Regulation

Negative feedback is when the product of a certain biological pathway inhibits a step in the pathway, thus reducing the amount of additional product created. Enzymes can be regulated via negative feedback.

Inhibition Types

There are various types of enzymes inhibitors. The end of the section provides a table summarizing what you need to know.

Competitive Inhibitor

• Competes with the substrate for the enzymes active site.
• Does not change the Vmax.
• Increases Km
• Requires more substrate to reach the Vmax, therefore can be overcome by high concentrations of the substrate.

Noncompetitive Inhibitor

• Binds at an allosteric site (not on the active site).
• Decreases Vmax
• Does not alter Km

Uncompetitive Inhibitor

• Binds to an allosteric site on the enzyme-substrate complex. Cannot bind before the substrate has been bound.
• Decreases Vmax
• Decreases Km

Mixed-Type Inhibitor

• Can bind to either the enzyme or the enzyme-substrate complex.
• Decreases Vmax.
• The Km can either increase or decrease.

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Inhibititor Category	Effect on Vmax	Effect on Km
Competitive	Unaffected	Increase
Noncompetitive	Decrease	Unaffected
Uncompetitive	Decrease	Decrease
Mixed-type	Decrease	Increase OR Decrease

Regulatory Enzymes

• Allosteric Enzyme: Generally a non-covalent and reversible binding of a molecule to the allosteric site of an enzyme (not on active site) which may increase or decrease enzyme function
• Covalentely-Modified Enzymes: The addition of a covalent group to the enzyme to regulate it activity. For example, phosphate groups can be attached to enzymes which can increase or decrease function.
• Zymogens: Zymogens are inactive forms of enzymes/proteins. When a protease cleaves a zymogen, it is turned into its active form.