Active Sites – What’s Happening?

Active Sites – What’s Happening? • 1. Substrate is bound • 2. A reaction is being catalyzed . . . . • e. g.

Mechanism of Action: 2 major types For HW- Skip #22.21 – too weird . . . 1. Lock-and-key model of enzyme mechanism. • 2. Induced-fit model

Mechanism of Action: 2 major types • 1. Lock-and-key model of enzyme mechanism. • The enzyme is a rigid three-dimensional body. • The enzyme surface contains the active site. 1. (key) NOTE: THE REACTION being Catalyzed 2. (lock) 1. 2.

Mechanism of Action • 2. Induced-fit model • The active site becomes modified • to accommodate the • substrate 1. Induced fit – substrate changes enzyme form/shape once in active site 2.

Mechanism of Action • Figure 22.9 - Competitive inhibition. • the inhibitor fits into the active site, thereby preventing the substrate from entering. Inhibitor got there 1st, Ha ha

Mechanism of Action • Figure 22.10 Noncompetitive inhibition. • the inhibitor binds to allosteric site, ( not active site) • This changes conformation of the active site. • The substrate no longer fits (properly). Alosteric site

See p. 562-3 Chem. Connections

Mechanism of Action • Lock-and-key model & Induced-fit model emphasize the shape of the active site. • However, Chemistry of active site is most important. • Just 5 amino acids participate in active sites of: ≤65% of the enzymes studies to date. • Examples: His > Cys > Asp > Arg > Glu. • Four have either acidic or basic side chains; the fifth has a sulfhydryl group (-SH).

Confirming your knowledge • Which of the following is correct describing the Induced-Fit Model of Enzyme action? Substrates fit into the active site: • b/c they are exactly the same size and shape • By changing their size and shape to match those of the active site 3. By changing the size and shape of the active site Upon binding

Challenge Question • Enzymes are long protein chains, > 100 Amino acids • The Active SITE contains only a few amino acids • Explain why other amino acids are present and what would happen to the enzyme activity if significant changes were made to the enzymes Structure.

Enzyme Regulation 1. Feedback control 2. Proenzymes 3. Isoenzymes

Enzyme Regulation 1. Feedback control: regulation process where the product (of a series of enzyme-catalyzed reactions) • inhibits an earlier reaction in the sequence. • The inhibition may be competitive or noncompetitive. (path 2) (path 1) (path 1) e.g. cholersterol normally produced in liver, [<100mg/100mL ] (path 2)if > [200mg/100mL] in plasma, liver stops producing . . .

Enzyme Regulation (cont.) • 2. Proenzymes (zymogens): inactive form of enzyme must have portion of polypeptide chain removed otherwise not active. • e.g. trypsin, (a digestive enzyme) • synthesized/stored as trypsinogen, no enzyme activity. • Active only after a six-amino acid fragment is removed • Removal of amino acid fragment changes primary + tertiary structure,  active form. • 3. Isoenzyme: different form of the same enzyme ( two different forms catalyze the same reaction(s))

Challenge Question • Why not make fully active form of Enzyme(s) all the time? (e.g. trypsinogen  trypsin)

The Allosteric Effect • Figure 22.12 Binding of regulator to a site other than the active site changes the shape of the active site.

Enzymes in Medicine • Enzyme assays useful in medical diagnosis. Monitor levels 24hr after M.I. or (heart attack)

Active Sites – What’s Happening?