Enzyme Catalysis Flashcards

This set of flashcards covers key vocabulary and concepts related to enzyme catalysis, including the regulation of enzyme activity, dietary protein breakdown, and protein degradation.

Enzyme Catalysis Overview

Enzyme activity is regulated and can be switched on (activated) or switched off (inhibited). This regulation can be via different mechanisms: Reversible binding of small molecules, Alteration of enzymes by covalent modification, Irreversible proteolytic activation, and Altering the total amount of an enzyme present.

Reversible Binding

Reversible inhibitors can be competitive or non-competitive depending on whether they bind to the active or a secondary site.

Reversible Binding - Competitive Inhibitors

Competitive inhibitors that bind at the active site are substrate analogues. This is not a common mechanism for controlling the activity of intracellular enzymes, but it is a common mechanism for drug action.

Reversible Binding - Allosteric Enzymes

Activation and inhibition of intracellular enzymes with allosteric properties is a general mechanism for controlling metabolism to meet the internal requirements of the cell. Many intracellular enzymes that are important control points in metabolism have allosteric properties.

Allosteric Enzymes - Mechanism

Allosteric enzymes oscillate between an active and an inactive conformation. Allosteric activators and inhibitors bind reversibly and lock the enzyme in the active or inactive conformation.

Reversible Binding – Allosteric Enzymes Profile

Allosteric enzymes display a different profile to other enzymes, showing a sigmoidal curve when reaction plotting the reaction velocity with varying substrate concentrations.

Reversible Binding – Control of glucose breakdown

The enzyme activity can be altered over a range of ~20 fold by the allosteric activator. Example: The enzyme Phosphofructokinase is a key regulator of glucose catabolism. F6P is the substrate. F2,6BP is the small molecule.

Reversible Covalent Modification

Alteration by covalent attachment of a modifying group. Commonly this is phosphorylation of hydroxyl (-OH) groups on amino acid side chains. This modification is reversible via dephosphorylation. This is like a molecule switch to turn activity “on” or “off.”

Reversible covalent modification- Cellular Response

Phosphorylation/dephosphorylation is not just common for regulating enzyme activity, but is as part of cellular responses to external signals, such as the binding or hormones or neurotransmitters to receptors, and transporters. This allows control of metabolism or other cellular processes in response to external signals.

Reversible Covalent Modification - Phosphorylation Details

Proteins are phosphorylated by kinases and dephosphorylated by phosphatases. Phosphorylation occurs on Serine, Threonine and Tyrosine. Phosphorylation adds two negative charges to a protein, altering its structure and function.

Proteolytic Activation

Some proteins are inactive when synthesised and require activation by specific proteolytic cleavage. The inactive precursor is called a zymogen or proenzyme. Activation occurs once only in the life of the protein (irreversible). Examples: Digestive enzymes, Blood clotting, Some hormones (e.g. insulin), Some structural proteins (e.g. collagen).

Proteolytic Activation in Digestion

Proteolytic activation of digestive enzymes breaks down dietary protein to provide intake of amino acids in Stomach and acidic environment using pepsin.

Proteolytic activation in digestion - Correct location

Gastric and pancreatic enzymes must not be active until they are in the correct location and in the environment where there are target proteins to digest.

Synthesis of zymogens

Zymogens are synthesized in the acinar cells of the pancreas. Hormonal signaling or nerve impulse causes their release into a duct leading to the duodenum.

Proteolytic Activation of Chymotrypsin

Trypsin cleaves chymotrypsinogen and results in Activation that changes the protein structure. The digestive proteins trypsin and elastase are homologues of chymotrypsin. Very different substrate specificities.

Substrate Specificity of Proteolytic Enzymes

Chymotrypsin has hydrophobic side chains, Trypsin has +vely charged side chains, and Elastase has small side chains.

Concurrent activation- Enteropeptidase

The cells lining the duodenum secrete enteropeptidase which cleaves and activates trypsinogen.

Reversible Binding in Digestion - Pancreatic Trypsin Inhibitor (PTI)

The pancreas also produces a pancreatic trypsin inhibitor (PTI) that acts as an off-switch for any trypsin formed by accidental trypsinogen activation. Accidental activation of trypsin is dangerous will lead to pancreatitis or tissue necrosis.

Summary: Proteolytic Activation in Digestion

Begins in the stomach where the acidic environment denatures the proteins, digested further by pepsin. Continues in the lumen of the intestine using proteolytic enzymes from the pancreas using Aminopeptidases in the plasma membrane of the intestinal cells to complete the digestion.

Enzyme regulation in digestion - Lipase and Colipase

Pancreatic lipase is secreted with colipase, a coenzyme, which binds to the inactive lipase and helps to bind it to its target. This causes a conformational change in the lipase, exposing an active site in the region of the enzyme surface in contact with the substrate and “switching on” the enzyme.

Controling Amount of Enzyme Present

This can be controlled through the regulation of gene expression or the degradation of existing cellular proteins.

Protein Degradation - Ubiquitin

Ubiquitin (Ub) marks proteins for degradation. Enables protein turnover to be tightly regulated. attaches to target proteins with the assistance of other enzymes.

Protein degradation - Proteasome

Ubiquitin places a target on the protein to be degraded; then The proteasome degrades the ubiquitinated protein.