Enzymes Protein catalysts- can accelerate reaction rates as much as 1017 – typical acceleration is 107/108 over uncatalyzed reactions Responsible regulating the rate of virtually all important chemical reactions in the cell (RNA can also catalyze splicing reactions) Highly specific – can distinguish between stereoisomer substrates; typically catalyze 1 reaction in the cell

Figure 6.15 The catalytic cycle of an enzyme

Models of Enzyme Action LOCK AND KEY INDUCED FIT Enzyme structure is rigid. Substrate is exact complement to active site shape of enzyme Enzyme structure changes upon binding of substrate Enzyme structure is flexible- currently accepted model. Enzyme binds substrate loosely, transition state tightly

Conformational changes in yeast hexokinase on binding glucose.

Figure 6.13 Enzymes lower the barrier of activation energy

How do Enzymes catalyze reactions?

Lactase Example of Biotechnology Lactase – enzyme that breaks down lactose (disaccharide) into Monosaccarides galactose and glucose Uses: Milk/dairy products for lactose intolerant people Use in milk/shakes or yogurt (galactose/glucose are sweeter and ice cream (lactose gives gritty texture when frozen. Bacteria ferment glucose and galactose more quickly than lactose reduces production time for yogurt and cottage cheese.

Factors that Affect the Rates of Enzyme catalyzed Reactions [Enzyme] - ↑ [E], ↑ rate [Substrate]- ↑ [S], ↑ rate only up to a point pH – optimal range Temperature- optimal range Inhibitors (↓) or activators (↑)

Enzyme Saturation

At low [S] concentrations, active sites are available; at high [S] are active sites are occupied, rate is limited by availability of enzyme

Figure 6.16 Environmental factors affecting enzyme activity

Effect of high temperature ↑Temperature → ↑ average KE (energy of motion) of atoms Distance between weakly attracted atoms in side chain ↑ → bonds break

Effect of Low Temperature Molecules move slowly, % of reactant substrate molecules with enough energy to overcome Ea ↓ Also, enzyme generally needs to be able adjust conformation upon binding of substrate(induced fit); lower KE could impact flexibility of enzyme ↓ catalytic efficiency

Effect of Changing pH Lower pH → ↑ [H+] → side chains become more + Higher pH → [H+] ↓ → side chains become more – Salt bridge and H-bonds disrupted

Figure 6.17 Inhibition of enzyme activity

Figure 6.19 Feedback inhibition