TemperatureTemperature

(I will focus on Adaptation of Enzymes)(I will focus on Adaptation of Enzymes)

Outline

(1) Physical and Physiogical Effects of Temperature (Q10)

(2) Evolution of Enzyme Function

Physical Forces in the Environment

Physical factors in the Environment (temperature, salinity, light, oxygen, etc) impose selective forces

to which the organism must respond

Physical Properties of Water and Air

Property Water AirHumidityHumidity High Low

DensityDensity High (800x) Low

ViscosityViscosity High (50x) Low

Heat CapacityHeat Capacity High (3000x)High (3000x) LowLow

OO22 Solubility Solubility Low High (30x)

OO22 Diffusivity Diffusivity Low High (8000x)

Light ExtinctionLight Extinction High Low

The range of temperature in water is less than…

The range of temperature on land

Range of temperatures on earth far exceed Range of temperatures on earth far exceed the range compatible with animal lifethe range compatible with animal life

OptimalTemperature

Rate of Reaction

orBiologicalProcess

Temperature

Rate Enhancing Effects

Destructive Effects

Q10 roughly describes

effects of temperature on physiology

Q10 = the effect of temperature

on physiology at one temperature versus another 10°C different

Q10 roughly describes effects of temperature on physiology

Higher Temperature

Increase in activity of molecules

Faster chemical reactions

Q10 roughly describes effects of temperature on physiology

Physiological processes could include metabolic rate, ingestion rate, digestion rate, etc…

= rate at T+10°C = rate at T rateT rateT-10°C

= ratio of the rate of a reaction at one temperature divided by the rate of the same reaction at a temperature 10 C° less.

Larger the Q10 = greater effect of temperature on rate of reaction.

Q10 = 1 implies no effect of temperature on the rate of reaction.

Typical Q10 values = 2 ~ 4

Q10

Q10 Q10 is only a very rough indication of the effect of

temperature on physiological activity

At greater temps, difference might be the same but ratio might decrease

– Temperature in Kelvins Ratio

T1/K T2/K k2/k1

273 283 2.00373 383 1.45473 483 1.26

But…

What other environmental variables might vary with temperature?

Important Point when thinking about environmental variables: Environmental variables can covary or interact with other

variables

For example, temperature covaries with a lot of other variables

Such as Viscosity, Oxygen concentration, pH, Solubility of a chemical, etc.

These other variables might also affect physiological processes

If you aren’t careful, effects of these other variables might be confused with effects of temperature

Important Point when thinking about environmental variables:

With increase water temperature, oxygen concentration declines (Charles’ Law)

With increasing temperature, CO2 concentration decreases, and blood pH increases

With increasing temperature, viscosity declines

When you think you are testing for the effects of Temperature, you might actually be measuring the effects of something else!!!

EXAMPLE: The coupling of EXAMPLE: The coupling of temperature with fluid viscosity temperature with fluid viscosity can greatly impact physiological can greatly impact physiological processes at small scales (low processes at small scales (low Reynolds Numbers)Reynolds Numbers)

So how much of Q10 is due to the effect of temperature alone, versus the effects of a covariable,

such as viscosity?

Separated Effects of Temperature and Viscosity by adding Dextran… dextran changes viscosity without changing temperature

Add Dextran to artificially raise viscosityIndependent of temperature

Relationship between

viscosity and temperature

Viscosity manipulated by adding dextran

Mean number of particles ingested over 10 minute trials

Temperature is 22°C, but viscosity is equivalent to that of 12°C (by adding Dextran)

About 60% of difference in performance is due to effects of Viscosity alone!!!!

Lesson

Many Physical Variables covary

When you are testing the effect of a variable (such as temperature) keep in mind that you could also be changing other variables

(such as O2 conc., viscosity, pH, etc)

Examine interaction term among variables in an analysis of variance (ANOVA)

Outline

(1) Physical and Physiogical Effects of Temperature (Q10)

(2) Evolution of Enzyme Function

EnzymEnzymeses

Terms Paralogs: genes related by duplication within a genome.

Following duplication, they often experience subfunctionalization, neofunctionalization, or loss of function

Orthologs: genes in different species that evolved from a common ancestral gene by speciation. Often, orthologs retain the same function during the course of evolution.

Isozymes: different forms of the same enzyme, usually resulting from gene duplications (paralogs); they often differ in amino acid sequence but catalyze the same chemical reaction. These enzymes usually display different kinetic parameters (i.e. different Km values), or different regulatory properties.

Allozymes: enzyme products of different alleles of the same gene (allelic enzymes at a locus)

Sample exam questionI have two closely related detoxification enzymes, that are nearby on the same chromosome. One breaks down cocaine and the other breaks down caffeine. These proteins are:

(A) paralogs

(B) orthologs

(C) isozymes

(D) allozymes

The Arrhenius Equation

The rate constant k of a chemical reaction depends on temperature T (in Kelvins) and activation energy Ea:

A = pre-exponential factor

R = gas constant

Ea = activation energy, minimum amount of energy required to transform reactants into products

k = A e-Ea/RT

• Enzymes lower the activation energy (Ea) of a chemical reaction (“catalyzes the reaction”)

• Different isozymes with different properties would lower the activation energy to differing degrees

• That is, enzymes with different Km or kcat will lower Ea to differing degrees

E + S E + PE Sk1 k2

k-1

Enzyme Reaction

E = enzyme

S = substrate

P = product

where

E S = enzyme-substrate complexk1 , k-1 , k2 = enzyme reaction ratesk2 is also called kcat, the catalytic constant

Michaelis-Menten Equation

Velocity (rate of reaction) =

Km = substrate affinity, where Vmax/2

Also called “Michaelis-Menten constant”

[S] = substrate concentration

Vmax = maximum velocity

Vmax [S]

Km + [S]

Michaelis-Menten Equation

Velocity (rate of reaction) =

• Small Km: enzyme requires only a small amount of substrate to become saturated. Hence, the maximum velocity is reached at relatively low substrate concentrations. (greater substrate binding specificity)

• Large Km: Need high substrate concentrations to achieve maximum reaction velocity.

Vmax [S]

Km + [S]

E + S E + PE Sk1 kcat

k-1

Enzyme Reaction

• There could be evolutionary differences in Km

• And kcat among species could evolve

• kcat depends on the G (activation free energy) of the chemical reaction

Catalytic Efficiency

Catalytic constant, kcat :

kcat = turnover number = the rate at which substrate is converted to product, normalized per active enzyme site; Et is the concentration of enzyme sites you've added to the assay

High kcat greater rate of reaction

The ratio of kcat / Km is a measure of the enzyme’s catalytic efficiency

Vmax

[E]t

kcat =

Adaptive Response of Enzymes

Evolutionary Shifts in

Reaction Norms

Km and kcat of A4LDH orthologs vary among species adapted to different temperatures

Fundulus heteroclitusFundulus heteroclitus

15°C difference in Mean Temperature 15°C difference in Mean Temperature along Atlantic Coastalong Atlantic Coast

LDH

Place and Powers, PNAS 1979

The two alleles of LDH have a latitudinal distribution

1° latitude change = 1°C change in mean water temperature

Enzyme function could evolve via changes in

STRUCTURE Amino acid composition (AA substitutions) Secondary, Tertiary, Quaternary structure

REGULATORY Protein expression (transcription, translation, etc) Protein activity (allosteric control, conformational changes,

receptors)

Review Lectures on adaptation

Adaptation of LDH-B to temperature

There are structural differences in the enzymes (in amino acid composition between a vs. b alleles, allozymes)

Differences in amino acid composition could result in functional differences in the enzyme

Enzyme kinetics of the allelic products (aa, ab, bb) differ in this case (that is, specific activity of the enzymes differ in different environments)

Place and Powers, 1979

aa genotype

abbb

kcat/Km is larger for the b allele at low temperatures

LHD-B b and a alleles have different catalytic efficiencies (kcat/Km) at different temperatures

Or they show what is called, “genotype by environment interaction”, i.e. different genotypes do different things in different environments

Place and Powers, 1979

aa genotype

abbb

LHD-B b and a alleles have different catalytic efficiencies (kcat/Km) at different temperatures

Or they show what is called, “genotype by environment interaction”

Fish

Significantly different rates of glucose uptake depending on whether the eggs were injected with the “a” versus “b” allele

The structural differences between the alleles seem to affect function

Weakness of this study?

DiMichele et al. 1991 Science

The Allozymes show differences in Function

But, differences in gene (protein) expression of the two alleles might also be important

Differences in gene expression could be caused by differences in the promoter, enhancer or some other regulatory element

NOT by differences in the nucleotide composition of the gene itself (by the amino acid composition of the protein)

Enhanced expression leads to greater number of copies of the gene being transcribed (and then translated into protein)

Figure: Transgenic FishRegulatory sequence (an enhancer) injected into Northern or Southern Fish

The regulatory sequence is contained within the 500, but not 400 base pair sequence

The Northern regulatory sequence enhances LDH activity when injected into both Northern and Southern fish (experiment performed at 20°C)

Schulte et al. 2000

control

Enhancer present

This difference in expression is due to the presence of a regulatory element (an enhancer)

Functional Tradeoffs

Functional capacity vs Enzyme stability

Cold vs Warm adapted enzymes

For many species (mammals, birds, reptiles, fish), orthologs of A4LDH of cold-adapted species are more effective at lowering activation energy (Ea values) than those of warm adapted species (Fields and Somero, 1998)

So then, why not have these more effective cold-adapted enzymes in all environments?

Fish

kcat values are higher in species adapted to colder temperatures

There are many possible limitations (costs or constraints) preventing complete adaptation to an environment (see paper by Somero)

One possibility is the tradeoff between functional capacity and enzyme stability

Tradeoff between functional capacity and enzyme stability

More cold-adapted enzymes are labile (flexible, higher kcat) and less stable at higher temperatures

If too unstable, lose geometry for ligand recognition and binding (higher Km)

Protein could become inactivated

Tradeoff between functional capacity and enzyme stability

Dark areas experience conformational changes during ligand binding, such that amino acid changes here could affect enzyme function (kcat or Km)

This Thr -> Ala amino acid substitution corresponds to temperate -> tropical shift

A4LDH

This Thr -> Ala amino acid substitution, at position 219 in the J-1G loop of A4LDH, corresponds to temperate -> tropical shift in Damselfish

Threonine is more hydrophilic and thought to make the loop more flexible (higher Km, kcat)

Threonine -> Alanine amino acid substitution at a catalytic loop corresponds to temperate -> tropical shift in Damselfish

Km and kcat are higher in the temperate (colder) ortholog

The Alanine amino acid substitution causes Km and kcat to be reduced in the tropical orthologs

Threonine is more hydrophilic and thought to make the loop more flexible

Johns and Somero 2004

Chromis caudilis (tropical, warmer)

Chromis punctipinnis (temperate, colder)

Chromis xanthochira (tropical, warmer)

Higher reaction rate in colder fish

Lower stability in colder fish

Km

kcat

Tradeoffs:

Colder (white circles): more flexible (high kcat), but loss of binding ability (high Km)

Warmer (black square, triangle): Less flexible (low kcat), but higher binding ability (low Km)

Johns and Somero 2004

Chromis caudilis (tropical)

Chromis punctipinnis (temperate)

Chromis xanthochira (tropical)

When cold, you need to compensate for lower rates of reaction activity by making the enzyme more flexible high kcat sacrifice Km (high Km)or, fast &sloppy enzymes;the cold will keep enzyme more stable

Higher reaction rate in colder fish

Lower stability in colder fish

Km

kcat

(1) Physiological responses tend to increase with (1) Physiological responses tend to increase with temperature, until they are limited by destructive effects temperature, until they are limited by destructive effects of high temperature of high temperature

(2) It is important to remember that many other physical (2) It is important to remember that many other physical variables covary with temperature such that experiments variables covary with temperature such that experiments can be confounded by multiple variablescan be confounded by multiple variables

(3) Enzyme activity is affected by temperature, and (3) Enzyme activity is affected by temperature, and enzymes can evolve function in response to enzymes can evolve function in response to temperature, such as altering amino acid composition, temperature, such as altering amino acid composition, conformation, or gene and protein expressionconformation, or gene and protein expression

(4) There are tradeoffs between enzyme lability and (4) There are tradeoffs between enzyme lability and stabilitystability

SummarySummary

Extra credit

Extra credit points will be given for sample exam questions from the course material

1-3 pts will be given for each question, for up to 6 points

For 2 questions

The questions must test thought and understanding, rather than simple regurgitation

Example of a question involving regurgitation:

Which of the following parameters indicates the substrate affinity of an enzyme?

(a) kcat (b) Km (c) Ea (d) Vmax

This is a good question but does not require an understanding of what the terms mean

This question would receive 0 points Also, 0 points for plagiarized questions, or those

identical to your classmates.

The following is a graph showing functional responses for four different enzymes (a, b, c, d).

14. Which of the enzymes has lowest substrate affinity? 15. Which of the enzymes has greatest catalytic efficiency (kcat / Km)?

Example of an exam question that tests

whether the student understands the

concepts

Email your extra credit assignment to:

carollee@wisc.edu by Saturday Feb 5, 5 pm

Put: “extra credit” in the subject heading

Make sure your name is clearly stated in the email

(1) Discuss Adaptations to high and low temperature at multiple hierarchical levels (amino acid substitution, gene duplications, etc).

(2) What is Q10? If oxygen consumption of an animal is 10 mol/s at 15°C, and 20 mol/s at 25°C, what is the Q10 of this physiological activity? What does this Q10 value mean?

(3) What other environmental variables interact with temperature, and how might a confounding physical variable affect the measurement of temperature effects on physiology (such as Q10)?

Study Questions

(4) What are the possible targets of selection for LDH in response to temperature?

(5) How does temperature affect Enzyme Kinetics?

(6) What changes in enzyme function might enhance a response to an environmental variable (such as temperature)? (Vmax, Km, Kcat, Kcat/Km, etc??)

(7) Why are there tradeoffs between enzyme function and stability?

(8) Why are there tradeoffs between cold and warm adaptation in enzyme function?

(9) Would global warming have the same or different effect on terrestrial versus aquatic organisms? Why? What about global cooling?