The_Scientist_-_December_2018

12.2018 | THE SCIENTIST 39

KERFELDLAB.ORG

Why house certain reactions in tiny containers? Computer

modeling indicates that microcompartments should maximize the

turnover of metabolites by keeping reaction intermediates close

and interfering chemicals at a distance.^7 For instance, concentrat-

ing the carbon-fixing enzymes RuBisCO and carbonic anhydrase in

a carboxysome makes the processing of carbon dioxide more effi-

cient.^8 And cordoning off toxic reactions, such as those that produce

aldehyde intermediates—a hypothesized, though unproven, job of

metabolosomes—would protect the rest of the cell’s interior.

Not all bacteria can make nano- and microcompartments.

In fact, most denizens of mammalian intestines seem to lack

microcompartments, although many pathogens possess them.

For example, Salmonella’s microcompartments metabolize the

organic compounds propanediol and ethanolamine, which are

found in processed foods and the human gut. Compartmentaliz-

ing the reactions is thought to allow the bacterium to digest nutri-

ents that members of the human intestinal microbiome cannot,

allowing the pathogen to outcompete them. Other pathogens,

such as Listeria and Clostridium, also contain metabolosomes.

Microcompartment biology

Bioengineers are eager to use bacterial compartments too (see

“Practical Applications” on page 42), and to do that, they’ll need a

better understanding of the organelles’ biology. Thanks to genet-

ics and cell biology experiments, “very basic design rules are start-

ing to fall into place,” says Danielle Tullman-Ercek, a chemical

engineer at Northwestern University in Illinois.

TYPES OF BACTERIAL COMPARTMENTS

Compartment type Function Size range

Carboxysome

These bacterial microcompartments contain RuBisCO and carbonic anhydrase,

allowing them to fix carbon efficiently by concentrating the substrate, CO 2 ,

and excluding O 2 , which can interfere by competing with CO 2 for RuBisCO. There

are two types: alpha-carboxysomes and beta-carboxysomes.

100–600 nm

Metabolosome

This type of microcompartment contains enzymes that break down certain

metabolites, allowing the structures to sequester toxic aldehyde intermediates

from the rest of the cell. There are several known types of metabolosomes.

40–150 nm

Encapsulin nanocompartment

These structures are smaller than microcompartments and are made of a different

type of shell protein. They appear to sequester toxins or store enzymes that help

bacteria deal with stressors. Scientists have identified at least three classes so far,

based on their associated enzymes.

25–66 nm

MICROCOMPARTMENT STRUCTURE: In 2017, Cheryl Kerfeld and colleagues

published the crystal structure of an assembled 40-nm microcompartment

from a myxobacterium. Kerfeld estimates the microcompartment could hold

approximately 300 interior proteins of 30 kiloDaltons each, though no one

knows for sure how such contents are organized.