Download Homeostasis Invertase

Homeostasis and Evolution - Tracking the effects of a single gene
Lab Overview:
Homeostasis is the maintenance of a stable constant internal state in the face of variation in
environmental conditions. These can be changes in temperature, water availability, or the loss of
a food source (there are many others). All organisms have mechanisms in place to regulate their
metabolic reactions in order to maintain homeostasis. You can think of this regulation at the level
of the body, at the level of the tissue (e.g. heart), at the level of the cell (e.g heart cell) or at the
level of the protein (e.g. enzyme). In fact, there is coordination across these levels to maintain
homeostasis of critical factors such as body temperature, ionic concentrations (like protons and
calcium), and "building blocks" like amino acids and nucleic acids.
From an evolutionary perspective, the average genotype of a population is the set of alleles that
produce a phenotype or phenotypes that allow individuals to manage homeostasis in their current
environment. If the environment does not change, then we can imagine two possible outcomes
for individuals with a new mutation producing a new phenotype in homeostasis. First, and more
likely, it could be disadvantageous: If individuals with the new phenotype must work harder to
maintain homeostasis, then they will be at a disadvantage relative to the "normal" types and this
phenotype should disappear. Alternatively, the new mutation could be advantageous: If
individuals with the new phenotype find it easier to maintain homeostasis, then they will be at an
advantage relative to the normal type and this phenotype should become more common. In both
cases, it is important to understand that the mutations giving rise to the phenotypic variation
happen by chance and not by necessity.
For each enzyme in an organism there is some fluctuation that can occur and still allow normal
function - the range that the homeostasis processes must maintain. In this lab, you will use the
enzyme invertase to investigate what changes in environmental conditions a protein can tolerate,
the range that the body must maintain. Use of a computer simulation will allow you to conduct
several different experiments to measure changes in invertase activity when you manipulate
temperature, pH, and substrate concentration and illustrate the degree of variation that can be
tolerated by the enzyme.
Metabolism and Enzyme Review:
Cells are complex chemical factories that break down and build up molecules important for the
maintenance of life. The sum total of these reactions is referred to as metabolism – breakdown
reactions are called catabolic (energy-yielding) and building reactions are referred to as anabolic
(energy-requiring). Often a breakdown reaction is spontaneous (i.e. runs downhill) while
anabolic reactions (i.e. run uphill) must be coupled to a reaction that can supply more energy
than is needed for the reaction to take place. However, even when reactions are spontaneous,
they often do not occur at a rate sufficient to maintain life. Thus some sort of catalyst is needed
for the reaction to occur; this is what an enzyme does.
Enzymes speed up (accelerate) very specific chemical reactions with incredible precision
because of the specific shape assumed by the enzyme protein. In catalyzing the reaction,
however, enzymes themselves are not altered by the reaction itself. They are the same in the
beginning and the end. After an enzyme has catalyzed a reaction, it releases the substrate and is
available to bind to another fresh substrate and repeat the reaction. Most enzymes will repeat this
cycle rapidly as long as there is a large amount of substrate.
All enzymes have a three-dimensional conformation producing the active site where the substrate
binds. The active site of an enzyme governs the specificity of the enzyme activity, and most
enzymes can only bind a single substrate. Binding of substrate to an enzyme forms an enzymesubstrate complex (ES). The enzyme then converts the substrate into a new molecule or
molecules called products (P). At the end, the enzyme releases the product(s) making the active
site free and available to repeat its cycle. The steps of a chemical reaction where an enzyme is
involved are seen in this equation:
E + S ES E + P
Enzymes are absolutely essential for accelerating biochemical reactions, and a number of
conditions influence enzyme activity. Most enzymes have a range of temperature and pH
conditions where their activity is highest. For example, blood enzymes perform optimally at a pH
close to 7.4, the pH of normal human blood, whereas stomach enzymes have an optimal pH of
around 2.0, the pH of normal stomach conditions. Varying these conditions typically affects the
conformation of the enzyme, which in turn influences an enzyme’s ability to bind to its substrate
and catalyze a reaction. Remember that enzymes can become denatured (disruption of 3-D
structure) in response to large changes in the environmental conditions (i.e. an increase in
temperature). Homeostasis is the set of biological processes that monitor and regulate the internal
conditions so that enzymes continue to function and vital biochemical reactions take place.
Invertase Background:
Now that you are familiar with some important biochemical properties of enzymes, it is time to
put your knowledge to work. In EnzymeLab you will work with an enzyme that most likely
played an important role in digesting some of the food molecules that you ate this morning for
breakfast! The enzyme chosen for this lab is invertase, also commonly called sucrase and
saccharase. This enzyme catalyzes the hydrolysis of the disaccharide sucrose, composed of a
monomer of glucose and a monomer of fructose. Invertase cleaves the bond between glucose and
fructose, producing the monosaccharides that can be utilized in cellular respiration.
Invertase is present in a wide range of organisms including animals, plants, yeast, fungi, and
algae. In humans, invertase is found in the membrane of a cell lining the inner walls of the small
intestine. Different organisms genes for invertase code for variants that have different
temperature and pH ranges of optimal activity: temperature varies from 40° C to 70° C and pH
varies from 4.0 to 10.0.
Laboratory Objectives:
You will use EnzymeLab Simulation to study important biochemical parameters of enzymecatalyzed reactions as illustrated by invertase. You will set up an experiment by adding substrate
to a virtual test tube along with purified enzyme to determine:
• The effects of changes in substrate concentration
• The temperature and pH optimums for invertase
This lab relies on computer modeling as a basis for the creation of the simulation. In most cases,
models allow scientists to perform experiments that would be very difficult to conduct otherwise.
The models scientists use are not just games or random number generators. Rather these models
are generated based on the knowledge learned from previous research, and carefully
programmed to produce the most realistic and meaningful data possible.
The computer model (simulation) in this teaching laboratory is based on the understanding of
enzymes generated over the last century. This type of program allows you to conduct
experiments that cannot be performed in lab due to time constraints. Go ahead and enter the
simulation and follow the instructions listed under the “assignments for non-majors”.
(Please note: The introductory text has been modified from the Student Lab Manual for Biology
Labs Online.)
Lab Discussion Questions
Remember, in reality these phenotype variations discussed in lab would result from random
mutations and not by necessity (meaning because invertase “needs” them).
1) Think about evolution and assume that the environmental conditions for invertase do not
change, what kinds of new phenotype variations might be advantageous (and selected
for)?
2) Think about evolution and assume that the environmental conditions for invertase do not
change, what kinds of new phenotype variations might be disadvantageous (and
eliminated)?
3) Invertase is found in a range of organisms, including some bacteria that are found as
human parasites and other bacteria that live in the soil. What homeostasis requirements
would you expect in each of these, and why?
4) Soil bacteria in Alaska are experiencing more changes due to climate change (warming)
than most soil bacteria. Given that mutations do not arise out of necessity, what kind of
evolutionary changes would you predict to find in these bacteria over the next 10 years?