Enzymes: Nature and Properties

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Questions and Answers

What are enzymes?

Enzymes are protein catalysts for chemical reactions in biological systems.

Why are enzymes important in the human body?

Enzymes lower the energy barrier for reactions, increasing their rate and allowing necessary cellular reactions to proceed at body temperature and pH.

What distinguishes a simple enzyme from a holoenzyme?

Simple enzymes consist only of protein. (correct)

Holoenzymes contain both protein and non-protein components. (correct)

Holoenzymes are made only of non-protein components.

Simple enzymes do not participate in catalytic activity.

What are isoenzymes?

Isoenzymes are enzymes with similar catalytic activity that act on the same substrate but differ in physical and chemical characteristics. Signup and view all the answers

What is the rate limiting step in a reaction?

The rate limiting step is the slowest step in a reaction, setting the pace for the entire reaction. Signup and view all the answers

What is an active site?

The active site is a special pocket or cleft in an enzyme where substrate binding and catalysis occur. Signup and view all the answers

Describe the efficiency of enzyme-catalyzed reactions.

Enzyme-catalyzed reactions are highly efficient, occurring 10^3 to 10^8 times faster than uncatalyzed reactions. Signup and view all the answers

What is absolute specificity in enzymes?

One enzyme acts on one substrate. Signup and view all the answers

What are Zymogens?

Inactive forms of enzymes that can be activated when required. Signup and view all the answers

Which of the following modifications can activate or inactivate enzymes?

Covalent Modification Signup and view all the answers

Feedback inhibition occurs at the committed-step enzyme.

True Signup and view all the answers

What are positive effectors?

Molecules that increase the catalytic activity of enzymes. Signup and view all the answers

What do you call substances that increase enzyme gene expression?

Inducers Signup and view all the answers

What is competitive inhibition?

A type of inhibition where the inhibitor competes with the substrate for the active site. Signup and view all the answers

Non-competitive inhibition decreases the Vmax of the enzyme.

True Signup and view all the answers

What is the role of inhibitors?

They decrease the velocity of an enzyme-catalyzed reaction. Signup and view all the answers

Which of the following represents a slow long-term regulation of enzyme activity?

Enzyme Synthesis Signup and view all the answers

Study Notes

Enzymes - Nature and Properties

Enzymes are biological catalysts, increasing the rate of chemical reactions in living organisms without being altered or consumed.
Enzymes are crucial for metabolic reactions, allowing them to proceed at a rate compatible with life.
Enzymes lower the activation energy of reactions, facilitating cellular processes that would otherwise be too slow.
Enzymes prevent unwanted reactions that require high activation energies from occurring.
Enzymes can be classified into:
- Simple enzymes: Composed solely of protein molecules.
- Holoenzymes: Composed of protein groups and a non-protein component, called a cofactor.
  - Cofactor: Non-protein component which can be organic (coenzyme) or inorganic (metal ions like Fe²⁺, Mn²⁺, or Zn²⁺).
  - Prosthetic group: A tightly bound cofactor that is difficult to remove without damaging the enzyme.

Isoenzymes (Isozymes)

Isozymes catalyze the same reaction on the same substrate, but differ in their amino acid composition, physical, and chemical characteristics.
They often originate from different sites within the organism.
A prime example is Lactate Dehydrogenase (LDH), which exists in five forms, each with four polypeptide chains (H = Heart, M = Muscle).

Rate Limiting Steps

The slowest step in any reaction is the rate-limiting step.
This step sets the overall pace for the entire reaction and typically occurs early in the reaction pathway.
The overall enzymatic reaction rate is dependent on the rate of the rate-limiting step.

Enzyme Properties

Active site: A specific pocket or cleft on the enzyme molecule where substrate binding and catalysis occur.
- The active site is formed by the folding of the protein, containing amino acid side chains.
- Substrate binding is non-covalent, forming an enzyme-substrate (ES) complex.
- The ES complex is converted to an enzyme-product (EP) complex before dissociating into enzyme and product.
- The induced-fit model describes a flexible binding interaction where the substrate induces a conformational change in the enzyme, resulting in a stronger binding site and optimized active site formation.
Efficiency: Enzymes are highly efficient, accelerating reaction rates by factors of 10³ to 10⁸ compared to uncatalyzed reactions.
- Enzymes typically process 100 - 1000 substrate molecules per second.
Specificity: Enzymes exhibit high specificity, interacting with only one or a few substrates and catalyzing a single type of reaction.
- Absolute specificity: An enzyme acts on only one substrate. For example, Glutamate Dehydrogenase only catalyzes the removal of the nitrogen group from glutamate, not other amino acids.

Regulation of Enzyme Activity

Enzyme activity can be regulated by several mechanisms, including zymogen activation, covalent modification, allosteric modulation, feedback inhibition, and enzyme synthesis.

Zymogen Activation

Inactive proenzymes or zymogens are activated by specific hydrolysis of peptide bonds at their site of action.
Many digestive enzymes and enzymes involved in blood coagulation are zymogens, examples include pepsinogen to pepsin, trypsinogen to trypsin, and plasminogen to plasmin.
Once activated, enzymes cannot revert back to their proenzyme form.
Zymogens serve as a protective mechanism preventing auto-digestion of tissues and intravascular blood coagulation.

Covalent Modification

Many enzymes are regulated by covalent modification, often through the addition or removal of phosphate groups on serine or tyrosine residues in their chain.
Phosphorylation can activate enzymes, while dephosphorylation can deactivate them.
This process results in two interconvertible forms of an enzyme: phosphorylated and dephosphorylated.

Allosteric Modulation

Some enzymes bind to small regulatory molecules called "effectors" at distinct "allosteric sites" other than their active sites.
Effectors can be positive or negative, increasing or decreasing the enzyme's catalytic activity, respectively.

Feedback Inhibition

A specific kind of allosteric regulation in which the end product of a metabolic pathway inhibits the activity of an enzyme involved in that pathway.
This occurs at the "Committed-Step Enzyme," the first irreversible and pathway-specific enzyme.

Enzyme Synthesis

Cells can regulate enzyme activity by altering the rate of enzyme synthesis, leading to changes in the total number of active sites.
This regulation is usually slow (hours to days) and occurs under specific physiological or pathological conditions.
Elevated insulin levels, for example, stimulate the synthesis of enzymes involved in glucose metabolism.
Substances increasing enzyme gene expression are called "inducers," while those decreasing it are called "repressors."
Enzymes constantly in use are usually not regulated by altering the rate of enzyme synthesis.

Inhibitors

Inhibitors are substances that decrease the velocity of enzyme-catalyzed reactions.
Competitive inhibition occurs when the inhibitor binds reversibly to the active site, competing with the substrate.
Competitive inhibitors are structurally similar to the substrate.
This type of inhibition can be reversed by increasing the substrate concentration.
Non-competitive inhibition occurs when the inhibitor binds reversibly to a different site on the enzyme than the active site.
Non-competitive inhibitors do not compete with the substrate.
This type of inhibition cannot be reversed by increasing the substrate concentration.
Uncompetitive inhibition occurs when the inhibitor binds reversibly to an enzyme-substrate complex, preventing product formation.
This type of inhibition can be reversed by increasing the substrate concentration.

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Description

This quiz covers the nature and properties of enzymes, including their role as biological catalysts in metabolic reactions. It explores classifications such as simple enzymes and holoenzymes, along with the importance of cofactors. Test your knowledge on how enzymes function and their significance in biological systems.