Download Energy/Chemical Energy in the Cell Chapter 5

Enzymes (B7)
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)
Compare inorganic catalysts vs. enzymes
(B.7.2)
• 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
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 velocity
at low concentration
needs higher concentration
to reach ½ velocity
Mechanism of enzyme action
(B.7.5)
Induced-Fit Animation
• 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
• 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