What is the Difference Between Enzyme Activity and Specific Activity

The main difference between enzyme activity and specific activity is that enzyme activity is the moles of substrate converted by the enzyme per unit time whereas specific activity is the activity of enzyme per milligram of total enzyme. Furthermore, enzyme activity measures the amount of active enzymes present under a given condition while specific activity measures the enzyme purity in the mixture.

Enzyme activity and specific activity are two enzyme units which measure the enzymatic activity. The measurement of enzymatic activity by enzyme assays is important for the study of enzyme kinetics as well as enzyme inhibition.

Key Areas Covered

1. What is Enzyme Activity
     – Definition, Units, Importance
2. What is Specific Activity
     – Definition, Units, Importance
3. What are the Similarities Between Enzyme Activity and Specific Activity
     – Outline of Common Features
4. What is the Difference Between Enzyme Activity and Specific Activity
     – Comparison of Key Differences

Key Terms

Enzyme Activity, Enzyme Purity, Enzyme Units, Specific Activity, Substrate Concentration

What is Enzyme Activity

Enzyme activity is the amount substrate converted by the enzyme in moles per unit time. The SI unit of the enzymatic activity is katal, which is equal to mol S^-1. However, this is a considerably large unit; hence, the most common unit used is the enzyme unit (U). 1U is equal to 1 μmol min⁻¹. Especially, the katal unit is assumed for the natural target substrate of the enzyme. However, enzyme activity can be measured for the standardized substrates such as gelatin or milk proteins as well. Here, the unit of the enzyme activity becomes gelatin digesting units (GDU) and milk clotting units (MCU), respectively.  

Figure 1: Enzyme Activity with Substrate Concentration

Moreover, substrate concentration has an effect on the enzyme activity. Enzyme activity increases with the increasing substrate concentration up to a certain point. After that, it becomes constant. In addition, a particular enzyme has an optimal pH and temperature where it shows the optimal activity. Therefore, the activity of an enzyme depends on given conditions. Hence, this measurement can determine the activity of the enzyme under given conditions.

What is Specific Activity

Specific activity is the activity of the enzyme per milligram of the total enzyme. It means that this measurement gives the purity of the enzyme in a mixture. Specific activity is equal to the enzyme activity divided by the mass of the total enzyme. Therefore, the common unit of the specific activity is μmol min⁻¹ mg⁻¹. The SI unit is katal/kg.

Figure 2: Enzyme Activity with the Percentage of Enzyme Available

The amount of active enzyme in the mixture can be determined by an active site titration. The specific activity of the enzyme at a given substrate concentration is a constant for the pure enzyme. Also, it is possible to determine the turnover number of the enzyme with the use of the molecular weight of the enzyme. Here, the turnover number of the enzyme refers to the number of catalytic cycles carried out by the enzyme per second.

Similarities Between Enzyme Activity and Specific Activity

• Enzyme activity and specific activity are two enzyme units which measure the enzymatic activity.
• Both measurements are important to determine the available enzymes in a mixture.
• Moreover, they are important in the study of enzyme kinetics and enzyme inhibition.

Difference Between Enzyme Activity and Specific Activity

Definition

Enzyme activity refers to the measurement of the quantity of active enzymes present and is dependent on conditions which should be specified while specific activity refers to the amount of substrate converted through enzymatic processes per unit of time and mass of the total. Thus, this is the main difference between enzyme activity and specific activity. 

Measurement

While enzyme activity is the moles of substrate converted by the enzyme per unit time, specific activity is the activity of enzyme per milligram of total enzyme.

Importance

Another difference between enzyme activity and specific activity is that enzyme activity measures the amount of active enzyme present under a given condition, specific activity measures the enzyme purity in a mixture.

SI Units

The SI unit of enzyme activity is katal while the SI unit of the specific activity is katal/kg. Hence, this is another difference between enzyme activity and specific activity. 

Common Unit

Also, the common unit of enzyme activity is U while the common unit of specific activity is μmol min⁻¹ mg⁻¹.

Conclusion

Enzyme activity is the amount of substrate converted by the enzyme in moles per unit time. It measures the amount of active enzyme present in a mixture at a given time. On the other hand, specific activity is the activity of the enzyme per mg of total enzyme. It measures the purity of the enzyme in a mixture. However, the main difference between enzyme activity and specific activity is the type of enzymatic activity measurement.

References:

1. Cornell, Brent. “Enzyme Activity.” BioNinja, Available Here

Image Courtesy:

1. “Michaelis Menten curve 2” Autor: Thomas Shafee – Vlastito djelo (CC BY 4.0) via Commons Wikimedia  
2. “Q10 graph c” By Thomas Shafee – Own work (CC BY 4.0) via Commons Wikimedia