BioChem Enzyme Regulation (11)

• presence of this enzyme in the blood stream is indicative of liver damage
• (catalyses the interconversion of alanine and pyruvate)

• LOW serum levels of alpha-1 antitrypsin is indicative of liver damage
• (normally secreted by liver)

Tissue damage

An inactive enzyme precursor

an enzyme's active site is blocked or its residues are out of alignment until a proteolytic cleavage at a specific activation site on the chain

By rapid proteolytic cleavage

abundant conversion enzyme in the blood coagulation cascade

performs two proteolytic cleavages which turn prothrombin into its active form, thrombin

• 1) calcium
• 2) phospholipids
• 3) Factor V
• 4) Factor X
• (3 & 4 are blood coagulation proteins)

Factor V cleaves prothrombin to prethrombin. Factor X cleaves prethrombin to thrombin.

formed after a single proteolytic cleavage by prothrombin-converting complex; a second cleavage between residues 15 and 16 by the same complex gives thrombin

• light chain residues = 1-15 don't change much; they're held to the rest of the enzyme (heavy chain) by a disulfide bond
• in the active enzyme, residue 16 reaches over the enzyme and forms a stabilizing interaction with residue 194
• this repositioning results in the proper formation of the catalytic triad, containing the active serene protease nucleophile residue, serine 195

IRREVERSIBLE

on the cell membrane surface but it's water soluble

Serine proteases

Glutamate

Thrombin

It allows for calcium binding, which is required to anchor prothrombin to the cell membrane

gamma-carboxylation is a post-translational modification that DOES NOT directly regulate enzyme activity, but IS required for formation of the active enzyme for blood coagulation

a cofactor required for the gamma-carboxylation of prothrombin

• a vitamin K analogue that is used to treat thrombosis (too much blood clotting)
• Vitamin K ANALOGUES interfere with the carboxylation reaction

It inhibits the gamma-carboxylation reaction of prothrombin, thereby reducing the number of prothrombin molecules attached to the cell membrane. This reduces the amount of active thrombin that can be synthesized by a cell.

calcium ions

an anti-coagulant that promotes the high-affinity binding of antithrombin to thrombin, thereby reducing blood clotting

thrombin is inactivated when the protein antithrombin binds to its active site

• a type of serine protease that breaks down elastin fibers; released by neutrophils into lung tissue
• unregulated elastase activity would result in emphysema

• it's an irreversible elastase inhibitor that prevents neutrophil-produced elastase from degrading elastin fibers in the lungs
• individuals with mutated alpha1-antitrypsin often have lung destruction and emphysema

• Cigarette smoke oxidizes a methionine residue on alpha1-antitrypsin involved in binding elastase
• methionine --> methionine sulfoxide
• Oxidation of this residue PREVENTS inhibition of elastase, results in over-degradation of lung elastin fibers --> lung scarring & emphysema

intravenous administration of alpha1-antitrypsin; protein diffuses from blood into lung, where it reaches therapeutic levels in the fluid surrounding epithelial cells

charge; if said amino acid residue (serine, threonine, or tyrosine) is in or near the active site of an enzyme, the enzyme activity MAY be changed

serine, threonine and tyrosine

phosphorylation, specifically in the activation loop of a kinase, leads to a 1000 fold increase in catalytic activity (eg. increases substrate binding)

• reversibly compete with a substrate for an enzyme's active site
• usually resemble the substrate in structure
• they are able to fit into the active site but cannot be acted upon by the enzyme
• eg. ethanol is a competitive inhibitor of ethylene glycol
• (binding site only)

competitive inhibition; V_max reached eventually but it takes much more substrate than usual

the more you use, the more COMPETITION there is with substrate

• Ki measures the affinity of an enzyme for a certain inhibitor
• the SMALLER the Ki, the GREATER the inhibitor's effectiveness

Competitive inhibitors increase Km but DO NOT alter Vmax

Km measures the affinity of an enzyme for its substrate. The smaller the Km, the greater the enzyme's affinity for its substrate. Km is also defined as 1/2 Vmax.

• bind to the enzyme in a location OTHER THAN the active site (whether or not a substrate is present) & affect the amino acid residues of the enzyme involved in the catalysis of the reaction
• (catalytic machinery only)

noncompetitive inhibition; V_max never achieved

Noncompetitive inhibitors LOWER Vmax but do NOT alter Km

CANNOT be reached, no matter how much substrate is added

• form very tight complexes, kill the enzyme by forming covalent bonds with it
• eg. DIFP

an irreversibly inhibitor that inhibits serene proteases, specifically the enzymatic hydrolysis of acetylcholine (NT) by acetylcholine esterase, thus acting as an effective nerve gas

related to half-life (hours)

the effective time relevant for recovery from irreversible inhibitors is the time required for re-synthesis of an enzyme

positive effectors that promote an enzyme to exist in its active form where it is a more effective catalyst

True

• sigmoidal 
• 

Feedback inhibition is a phenomenon by which the end product of a reaction or sequence of reactions inhibits the enzyme or enzymatic cascade. When sufficient product is produced, the system automatically turns itself off.

• enzyme responsible for phosphorylating fructose-6-phosphate (using ATP) in the committed step of glycolysis
• -is allosterically activated by AMP (signals that cell is energy poor)
• -is reversibly inhibited by ATP (product of glycolysis)

• 1. They are functionally irreversible
• 2. They occur early in the enzymatic pathway
• 3. They occur in only one pathway

Cytidine triphosphate (CTP)

Adenosine triphosphate (ATP)