Enzyme Kinetics

Reaction Progress Curve

• Reaction progress is determined by changes in the free energy of the substrates (S), transition state, and products (P).
• Free energy can be calculated via the Gibbs free energy equation: ΔG = ΔH – TΔS
  • ΔG: change in free energy
    • Lower values mean that the reaction is more likely to occur.
    • Negative values mean that the reaction will occur spontaneously.
  • ΔH: change in enthalpy
    • Associated with changes in heat Heat Inflammation of a reaction in which exothermic reactions have high enthalpy and release heat Heat Inflammation
    • Endothermic reactions have low enthalpy and require heat Heat Inflammation
  • T: current temperature of the environment
  • ΔS : change in entropy
    • Entropy is a measure of the disorder of a system, which is dependent on the number of possible microstates.
    • Example: systems with a ↑ number of possible microstates → ↑ entropy
• The transition state represents the most unstable point of interactions between the enzyme and the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes(s) and is the highest energy point of the reaction.
• Enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes stabilize the transition state and lower the activation energy (ΔG⁰) of the reaction, making the reaction significantly easier to achieve.
• Once the transition state is attained, the reaction must proceed to a lower energy state. This is accomplished by either converting the substrates to products or reverting back to the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes form.

Dependence of the Reaction Rate on Temperature

• A ten-degree Centigrade rise in temperature will increase the activity of most  enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes by 50 to 100%, and variations as small as 1 or 2 degrees may increase the activity by 10 to 20%.
• This can only take place within a limited range of temperatures since enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes have an optimum temperature and they become denatured (irreversibly degraded) at temperatures that are too high.
• Common body temperatures:
  • 36–38^oC: Optimum body temperature Body Temperature The measure of the level of heat of a human or animal. Heatstroke; allows enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes to function properly.
  • < 36^oC: Hypothermia Hypothermia Hypothermia can be defined as a drop in the core body temperature below 35°C (95°F) and is classified into mild, moderate, severe, and profound forms based on the degree of temperature decrease. Hypothermia causes dramatic slowing of enzyme function.
  • > 38^oC: Fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever can initially allow enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes to function better.
  • > 40^oC: Hyperthermia causes enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes to begin to denature and lose function ( heat Heat Inflammation exhaustion and heat stroke Heat stroke A condition caused by the failure of body to dissipate heat in an excessively hot environment or during physical exertion in a hot environment. Contrast to heat exhaustion, the body temperature in heat stroke patient is dangerously high with red, hot skin accompanied by delusions; convulsions; or coma. It can be a life-threatening emergency and is most common in infants and the elderly. Body Temperature Regulation).

Dependence of the Reaction Rate on pH

• Enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes operate at an optimum pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance. Changes in the pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance value cause changes within the functional groups of the enzyme or of its substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes.
• Changes in pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance lead to changes in the ionic or hydrophobic forces that affect the spatial structure at the active center and improve or deteriorate the enzyme’s ability to bind BIND Hyperbilirubinemia of the Newborn its substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes.
• Stronger pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance changes in either direction may even denature the enzyme. Pepsin Pepsin Pepsin breaks down proteins into proteoses, peptones, and large polypeptides. Proteins and Peptides, for example, works effectively at a pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance of about 2, where other enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes would long have been denatured.
• Enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes in different areas of the body operate at the most common pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance values of those areas.
• Common pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance values:
  • Bloodstream: 7.35–7.45
  • Stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy: < 2 when full; 2–6 when fasting
  • Duodenum Duodenum The shortest and widest portion of the small intestine adjacent to the pylorus of the stomach. It is named for having the length equal to about the width of 12 fingers. Small Intestine: Anatomy: 5–7
  • Jejunum Jejunum The middle portion of the small intestine, between duodenum and ileum. It represents about 2/5 of the remaining portion of the small intestine below duodenum. Small Intestine: Anatomy: 6-7
  • Ileum Ileum The distal and narrowest portion of the small intestine, between the jejunum and the ileocecal valve of the large intestine. Small Intestine: Anatomy: 7-8

Dependence of the Reaction Rate on Substrate Concentration

Steady state conditions

Early changes in concentrations of S, enzyme (E), enzyme-substrate complex Enzyme-substrate complex Temporary molecule formed by the non-covalent binding of the enzyme and substrate. Basics of Enzymes (ES), and P change dramatically and are difficult to measure. Steady state occurs when changes in E and ES are relatively small.

• E and S are high early in the reaction, while ES and P are low.
• ES and P increase as the reaction proceeds, decreasing E and S.
• As S decreases, so does the formation of ES late in the reaction. At this point, a reverse reaction becomes likely.

Initial reaction rate (V_o)

The initial rate of the reaction is used to avoid the measurement of the reverse reaction once enough product has been made.

• Higher substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes concentrations increase V₀ until the reaction approaches V_max.
• V₀ = V_max[S] / K_M + [S]
• As [S] goes up, V₀ approaches V_max.
• As [S] goes down, V₀ approaches K_M.
• K_cat = the amount of product produced when the reaction achieves V_max. This allows for the measurement of concrete results of a reaction instead of rates.

Michaelis-Menten graph

Plotting the initial reaction rate (V₀) on the y-axis against the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes concentration on the x-axis on a graph results in a hyperbolic curve, which approaches the maximum velocity V_max at high substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes concentrations due to saturation of the enzyme with substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes.

Michaelis-Menten constant (K_M)

K_M is the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes concentration at which half-maximal velocity (½ V_max) is reached (K_M is measured on the x-axis while ½ V_max is measured on the y-axis).

• Indicates the affinity of an enzyme for its substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes in an inverse manner and is characteristic for the particular enzyme-substrate complex Enzyme-substrate complex Temporary molecule formed by the non-covalent binding of the enzyme and substrate. Basics of Enzymes
• If the K_M value is low, the enzyme has a strong affinity for the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes and less is needed to reach ½ V_max.
• If the K_M value is high, the enzyme has less affinity for the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes and more is needed to reach ½ V_max.
• Dependent on the temperature and pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance value, but independent of the enzyme concentration

Lineweaver-Burke plot

1/V₀ is plotted on the y-axis and 1 / [S] is plotted on the x-axis, resulting in a linear plot of the same data used in Michaelis-Menten kinetics.

• The slope of the line is equal to K_M / V_max.
• The y-intercept is equal to 1 / V_max.
• The x-intercept is equal to -1 / K_M.