Download Enzymes and Active Sites

Lecture Presentation
Chapter 20
Enzymes and
Vitamins
Karen C. Timberlake
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chapter 20 Enzymes and Vitamins
A physician assistant (PA)
helps a doctor by examining
and treating patients as well
as prescribing medications.
His or her duties may include
obtaining patient medical
records and histories,
diagnosing illnesses,
educating and counseling
patients, and referring the
patient, when needed, to a
specialist.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chapter 20 Readiness
Core Chemistry Skills
• Interpreting Graphs (1.4E)
• Identifying the Primary, Secondary, Tertiary, and
Quaternary Structures of Proteins (19.4, 19.5)
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
20.1 Enzymes and Enzyme Action
Enzymes are biological
catalysts that
• increase the rate of a
reaction by changing
the way a reaction
takes place.
• are not changed in the
process of the reaction.
• lower the activation energy of
the reaction.
Learning Goal Describe
enzymes and their role in
enzyme-catalyzed reactions.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
The enzyme carbonic anhydrase lowers
the activation energy for the reversible
reaction that converts CO2 and H2O to
bicarbonate and H+.
© 2016 Pearson Education, Inc.
Enzymes Lower Activation Energy
Enzymes increase the rate of a chemical reaction by
reducing the energy required to convert reactant
molecules to products.
An enzyme in the blood called carbonic anhydrase
catalyzes
• the rapid interconversion of carbon dioxide and water to
bicarbonate and H+.
• the reverse reaction, converting bicarbonate and H+ to
carbon dioxide and water.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Enzymes and Active Sites
Nearly all enzymes
• are globular proteins with a unique three-dimensional
shape that recognizes and binds a small group of
reacting molecules, called substrates.
• have a tertiary structure that includes a region called
the active site where one or more small groups of
substrates bind to create a chemical reaction.
• have specific amino acid residues within the active site
that interact with functional groups of the substrate to
form hydrogen bonds, salt bridges, and hydrophobic
interactions.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Enzymes and Active Sites
Enzymes like lactase have an active
site where the substrate fits for
catalysis to occur. The quaternary
structure of lactase consists of four
subunits. The substrate, lactose
(gray), is held in place in the active
site by hydrogen bonds with amino
acid side chains.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Specificity of Enzymes
• Some enzymes show absolute specificity by catalyzing only
one reaction for one specific substrate.
• Other enzymes catalyze a reaction of two or more
substrates.
• Some enzymes catalyze a reaction for a specific type
of bond.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Enzyme-Catalyzed Reaction
• The combination of an enzyme and a substrate forms
an enzyme–substrate (ES) complex.
• The ES provides an alternative pathway for the reaction
with lower activation energy.
• Within the active site, amino acid R groups catalyze the
reaction to form an enzyme-product (EP) complex.
Core Chemistry Skill Describing Enzyme Action
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Enzyme–Substrate Complex
A flexible active site in
lactase and the flexible
substrate lactose adjust
to provide the best fit for
the hydrolysis reaction.
Once the disaccharide is
hydrolyzed, the
monosaccharide products
are released from the
enzyme, which is ready
to bind another lactose.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Models of Enzyme Action
• A lock-and-key model has a rigid substrate binding
to a rigid enzyme, much like a key fitting into a lock.
• The induced-fit model, a more dynamic model of
enzyme action, states that the active site is flexible
enough to adapt to the shape of the substrate.
• The induced-fit model has the substrate and enzyme
working together to acquire a geometrical
arrangement that lowers the activation energy.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
1. Which is the active site?
A. the entire enzyme
B. a section of the enzyme
C. the substrate
2. In the induced-fit model, what happens to the
shape of the enzyme when the substrate binds?
A. stays the same
B. adapts to the shape of the substrate
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
1. Which is the active site?
B. a section of the enzyme
2. In the induced-fit model, what happens to the
shape of the enzyme when the substrate binds?
B. adapts to the shape of the substrate
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.