biochemaholic.wordpress.com

INTRODUCTION :

Enzymes are proteins that catalyze reactions that are

necessary for life.

They are composed of strings of amino acids, and the

particular sequence of amino acids determines what three-

dimensional shape each protein has, and what enzymatic

function it carries out.

Biologists categorize enzymes into families based on similarity

of structure. The more similar the structure, the closer the

evolutionary relationship is presumed to be.

BASIC CONCEPT :

streakscienceweekly.wordpress.com

HOW DID THEY EVOLVE?

There are 2 theories proposed to explain the evolution of

enzyme systems:

Retrograde

Patchwork

RETROGRADE :

In 1945 one of the first theories regarding the evolution of

metabolic pathways, often referred to as the retrograde

evolution model, was proposed by Horowitz.

It states that :

Pathways evolve backwards: the end

product of the newly evolved reaction is the substrate of the existing one.

Picture adapted from Betts & Russell, 2009

E 1

B A

Picture adapted from Betts & Russell, 2009

E1 E2

A B X

Successive reactions in the pathway would therefore be

catalysed by homologous enzymes.

Picture adapted from Betts & Russell, 2009

E2 E1

X A B

PATCHWORK

In 1976 Jensen proposed the recruitment evolution theory,

more often referred to as the patchwork evolution model .The

patchwork evolution model states that

“Enzymes initially have broad substrate

specificities and that specialization takes place

by way of gene duplication”.

E1

A

A’

E1 E3

A

A’

E2

IN FAVOR OF RETROGRADE EVOLUTION MODEL

Saqi & Sternberg showed that some super-families have a

general tendency to appear twice or more in one particular

pathway(s).

Ex : Deoxyribonucleoside kinases, which catalyse the

phosphorylation of deoxyribonucleosides, are present in

several copies in most multicellular organisms and therefore

represent an excellent model to study retrograde evolution.

CONT…….

Rison et al showed that homologous enzymes are found at close

distances within the (extended) pathways of E. coli .

Alves et al showed that homologous enzymes are also found close

to each other in the whole metabolic network.

pubs.rsc.org

IN FAVOR OF PATCHWORK

The patchwork evolution model holds that there should be many pairs of homologous enzymes that catalyze basically the same kind of reaction, where one or more substrates are non-identical but similar.

Support for this theory is more abundant than for the retrograde evolution model .

The TIM-barrel containing enzymes have been found in many different pathways .

The homologous pairs of small molecule metabolism enzymes of E. coli have been shown to be evenly distributed within and across pathways

blogs.yis.ac.jp

E zy es ould ’t e ol e

*The investigators compared the three-dimensional structures of similarly shaped enzymes that are found in different species of bacteria:

One enzyme splits water and combines the resulting hydrogen atoms with sulfur . (CS2 hydrolase]

Another class of enzymes that also splits water molecules, but then combines the hydrogen with a carbon-based molecule.

*The core structure of the CS2 hydrolase like that of similar enzymes, is critical. The scientists wrote in Nature, "Any change in this area of the protein [enzyme] adversely affected protein activity."

*The researchers also found that CS2 hydrolase is distinct from enzymes with an otherwise identical core because it has an additional long, narrow tunnel through which only CS2 can pass. The tunnel "functions as a specificity filter," ensuring that no similar molecule such as carbon dioxide enters.

Co t… • The DNA that codes for the tunnel portion of the CS2hydrolase gene is surrounded

by unique sequences, indicating that this DNA portion may have been added to the main enzyme's DNA. Perhaps some unknown cellular mechanism "stitched in" this extra bit at just the right place among the bacteria's 1.8 million DNA bases, adding the tunnel portion to a CO2-converting enzyme and thereby forming CS2 hydrolase.

Arguments : • If the gene jumped from another bacterium to this one, it did not

evolve because it already existed elsewhere.

• In addition to the enzymes themselves, another mechanism had to already exist that could recognize, accept, and insert the foreign DNA in just the right place. Only then could it retrofit an enzyme in just the right way to enable the bacterium to live on sulfur.

• This ignores the facts that no new DNA actually "emerged," and the proper placement of transferred DNA required just the opposite of evolution—purposeful design.

Conclusion from their research :

**CS2 hydrolase did not evolve. In fact, experimental science

shows that this enzyme functions today only because of its

precise and specific arrangement of parts. And like any

machine with multiple, interconnected parts, whether

biological or man-made, all the correct parts assembled in the

correct configuration were needed from the very beginning.

champperformance.com

The theories of evolution are still under

the process of evolution

*The whole concept can be summarized by the fact that the 2 theories of evolution of

enzymes, proposed are not universally accepted; i.e. there are exceptions to the

theories (ex : CS2 hydrolase).

*Scientists are still working on the theories to make them more universally acceptable.

References

Research articles • Net ork a alysis of eta oli e zy e e olutio

inEscherichia oli” y Sara Light and Per Kraulis.

• Ne “tudy “ho s E zy es Could 't E ol e y Bria Thomas, M.S.

Web links :

• http://www.biomedcentral.com

• http://www.evolutionnews.org/