Enzymes (B7)

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Characteristics (B.7.1)

• globular proteins that act as biological catalysts• lower the activation energy

• speed up reactions (up to 108 – 1012 faster) to a rate that is useful by the cell

• each enzyme reacts (temporarily binds) with a specific molecule in the reactants called a substrate

• have no effect on (ΔG) of a given reaction• can be catabolic (break molecules down) or anabolic

(build molecules up)

• both speed up reactions by providing an alternative pathway of lower activation energy– neither changes the position of equilibrium or

yield of the reaction• enzymes are highly specific for their substrate,

whereas inorganic catalysts are often non-specific and can catalyze several reactions

• enzymes have an optimum temperature and are denatured at high temperatures, whereas inorganic catalysts are much less a affected by the conditions and generally work well at high temperatures

Compare inorganic catalysts vs. enzymes (B.7.2)

Substrate concentration and enzyme activity B.7.3

• substrate concentration – as substrate concentration increases, the probability of enzyme colliding with substrate increases and therefore increases rate of reaction

• enzyme saturation is reached when all active sites on the enzyme are engaged

Vmax and Michaelis constant (Km) graphs B.7.4

• the curve when the rate for an enzyme saturated with substrate is known as the maximum velocity (Vmax)

– is the maximum rate at which an enzymatic reaction is occurring

– Vmax varies from one enzyme to another and is dependent on reaction conditions such as temperature and pH

• Michaelis constant (Km)

– this is the concentration of substrate when the rate is equal to one half Vmax

– enzymes with low Km

• indicates that it has a high affinity for a substrate, as only a small concentration of substrate is needed for the reaction to proceed at half its maximum velocity

– enzymes with high Km

• indicates that the enzyme has less affinity for the substrate, as a large concentration of substrate is needed to reach half Vmax

already at ½ velocityat low concentration

needs higher concentration to reach ½ velocity

Mechanism of enzyme action (B.7.5)

• enzymes contain an active site which is a specific location on the enzyme where reactions occur

• generally, a given enzyme is able to catalyze only a single chemical reaction or, at most, a few reactions involving substrates sharing the same general structure

• enzyme and substrate meet by random motion of molecules and connect

Induced-Fit Animation

• the “connection” is made by weak ionic and hydrogen bonds between areas on the substrate and enzyme

• the enzyme undergoes slight change in shape bringing reactive amino acid R-groups in the enzyme closer to the substrate (this is called induced fit) which can lower the activation energy by: – physically stressing the substrate which helps break the

bonds– sometimes changes pH within the active site– even can temporarily covalently bond with the substrate

• the entire reaction from binding to release occurs thousands to millions times a second

Mechanisms of Enzyme Control (B.7.6)

• two common ways of interference:• competitive• noncompetitive inhibition

1. Competitive Inhibition • molecule that is similar to the usual substrate

binds to the active site of the enzyme instead of the substrate

• adding more substrate helps (increase the rate) since there is increased competition for the active sites by the substrate

Figure 2. Normal activity of an enzyme substrate complex (2A). Herbicide binding to the enzyme and preventing normal action (2B). The enzyme in a herbicide resistant plant that can't bind with the herbicide (2C).

2. Noncompetitive Inhibition • chemical binds to the enzyme at area other than the

active site • this alters the enzyme’s shape enough so the

substrate doesn’t fit well, or at all, and therefore the rate of reaction slows down or stops

• adding more substrate does not help (increase the rate) since the active site is non-functional

Enzyme Activity (B.7.7)

• enzymes can become denatured (changes shape, and substrate is no longer functional)– heavy metal ions – poison enzymes especially by

reacting with S-H groups– pH – each enzyme has an optimal pH at which

they perform– temperature – body temp. (37°C ideal for

humans) • extreme heat will denature proteins