Chapter 4: Enzymes, Energy, and Metabolism
Lecture Outline
Introduction
A. Mechanism of Enzyme Action
B. Factors Affecting Enzyme Action
C. Bioenergetics
D. Cellular Respiration
1. Anaerobic Respiration
2. Aerobic Respiration
E.
Introduction
At the most fundamental level, life is the sum of a complex web of inter-related biochemical reactions taking place in each living cell of the body.
The general term used to describe these biochemical reactions is metabolism.
Metabolism: the sum total of the biochemical changes that occur within a cell = metabolic reactions.
Enzymes are tiny protein-based “molecular machines” (catalysts) which increase the rates of metabolic reactions in order to support the requirements of life.
Basic reaction characteristics of enzymes:
1) increase the rate of reactions by lowering activation energy (Ea) (fig. 4.1 p 80)
2) enzyme is not altered by the reaction
3) enzyme does not alter products of the reaction
A. Mechanism of Enzyme Action
All enzymes have an active site within their tertiary protein structure (fig. 4.2 p. 80).
The active site is where the metabolic reaction is catalyzed according to the following steps:
1. substrate molecule(s) + enzyme active site
2. enzyme-substrate complex (lock-and-key model)
3. enzyme + reaction product molecules
B. Factors Affecting Enzyme Action
1) Temperature (fig. 4.3 p. 81)
High temperatures denature proteins
Denature = breaking H-bonds (not the stronger covalent bonds) which disrupts tertiary and secondary protein structure = the structure of the molecular machine is broken
Therefore enzyme becomes inactive
2) pH (H+ concentration) (fig. 4.4 p. 82)
Changes in pH effect the properties of the amino acid functional groups
Therefore enzyme becomes less efficient or even inactive
3) cofactors (inorganic) and coenzymes (organic) (fig. 4.65p. 82)
Cofactors = usually ions
Coenzymes = several types of vitamins
Assist the enzyme in catalyzing reactions
4) substrate concentration (fig. 4.6 p. 83)
As substrate concentration rises enzyme activity rises up to a peak called the saturation point at which all enzymes are working at the maximum possible rate
Further increase in substrate concentration do not increase reaction rate
5) reversible enzyme reactions
Some reactions catalyzed by enzymes (but not all) are reversible
The direction in which the reaction proceeds is dependant on the relative concentrations of substrate and product molecules
6) metabolic pathways
Most enzymatic reactions are part of metabolic pathways in which an initial substrate passes through several intermediate products before the final product is formed
Very often these metabolic pathways have branches so that intermediate products can be used by other metabolic pathways
C. Bioenergetics
Bioenergetics is the study of energy management in living organisms.
In this section we will look at one of the fundamental metabolic reactions in all living cells = cellular respiration: the release of energy from organic molecules to use for the energy-consuming activities of the cell eg. transcription, translation, cell division, etc. (fig. 4.12 p. 86).
1st law of thermodynamics (energy)
Energy can be transferred, but not created or destroyed.
2nd law of thermodynamics (structure)
Entropy (measure of disorder) increases unless energy is invested.
In terms of energy and structure, 2 types of metabolic reactions occur in living cells:
1) Endergonic rnxs.
energy consuming
anabolic = building structure
2) Exergonic rxns.
energy releasing
catabolic = breaking down structure
ATP (adenosine triphosphate) serves as the link between exergonic and endergonic rxns. occuring in the body (fig. 4.14 p. 87).
Most endergonic reactions of the body use ATP as their energy source.
ADP + Pi -----> ATP
Requires input of 7.3 kcals per mole (endergonic / anabolic reaction).
Energy required for this reaction comes from specific organic molecules via cellular respiration.
ATP -----> ADP + Pi
Releases 7.3 kcals per mole (exergonic / catabolic reaction).
(the equivalent amount of energy required to make a covalent bond is released when the same covalent bond is broken)
Energy released from this reaction is used to drive endergonic / anabolic reactions of the body = eg. transcription, translation, cell division, muscle cell contractions, etc.)
D. Cellular Respiration: releasing energy from organic molecules
The primary exergonic (catabolic) reaction of the body.
Involves the breakdown (oxidation, combustion) of organic molecules.
The body uses several organic molecules as sources of energy:
1) carbohydrates - glucose
2) lipids -glycerol, fatty acids, ketone bodies
3) proteins - amino acids
Covalent bonds within these organic molecules are broken by enzymes which remove high energy H atoms = oxidation.
These high energy H atoms are temporarily transferred to a number of carriers = reduction.
The final H atom acceptor is oxygen.
Energy is released during the transfer process and used to form ATP (40% effeciency).
Cellular respiration can be divided into two distinct phases:
1) Anaerobic respiration (glycolysis)
O2 not necessary
2) Aerobic respiration (Krebs cycle)
O2 necessary
We will first look at the oxidation of glucose. In section E we will look at the oxidation of lipids and proteins.
1. Anaerobic Respiration (glycolysis) (fig. 4.19 p. 91)
Occurs in the cell cytoplasm.
O2 is not necessary.
Glucose is broken down to pyruvic acid.
2 ATP are formed (substrate-level phosphorylation).
2 NADH + H+ are formed.
glucose + 2 NAD + 2 ADP + 2 Pi -----> 2 pyruvic acid + 2 NADH + 2 ATP
If O2 is not present in sufficient quantities then the oxidation of glucose stops with the formation of pyruvic acid.
Lactic acid is formed from pyruvic acid to regenerate NAD in order to keep glycolyis going (fig. 4.20 p. 91)
2. Aerobic Respiration (Krebs cycle and the electron transport chain)
Occurs in the mitochondria.
O2 is necessary.
Pyruvic acid produced during glycolysis is 100% oxidised during aerobic respiration.
Consists of 2 processes:
a) Krebs cycle
b) electron transport system (ETS)
a. Krebs Cycle (fig. 4.25 p. 95)
1 ATP is formed (substrate-level phosphorylation).
3 NADH + H+ and 1 FADH2 are formed.
b. The Electron Transport System (ETS) (fig. 4.26 p. 96)
The chemical energy present within NADH + H+ and FADH2 formed during glycolysis and Krebs cycle are used to drive the production of ATP through oxidative phosphorylation.
1 NADH + H+ = 3 ATP.
1 FADH2 = 2 ATP.
O2 acts as the final electron acceptor.
Total ATP produced during cellular respiration for 1 glucose monosaccharide:
30 ATP.
E. The metabolism of lipids and proteins (fig. 4.35 p. 104)
The cells of the body can also use lipids and proteins to free energy for ATP production.
1) Excess carbohydrate in the diet is converted to triglyceride and stored in adipose tissue = lipogenesis.
2) Stored triglyceride can be released from the adipose cells as glycerol, fatty acids, and ketone bodies when the body needs energy and these molecules can then enter cellular respiration for energy release to form ATP = lipolysis.
3) Amino acids can also be used for energy release to form ATP, but this is not a favored pathway because we do not have a true store of amino acids, they are all involved in body structure (muscle, connective tissues etc.) and function (enzymes, transport proteins etc.).
Deamination precedes the use of amino acids in cellular respiration.
4) However, at times when dietary carbohydrate intake is low, the body will use amino acids to form new glucose for the brain = gluconeogenesis.