The monosaccharides glucose thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body.
The steps in carbohydrate digestion are summarized in Figure 1 and Table 1. Figure 1. Digestion of carbohydrates is performed by several enzymes. Starch and glycogen are broken down into glucose by amylase and maltase. Sucrose table sugar and lactose milk sugar are broken down by sucrase and lactase, respectively. A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids.
In the duodenum, other enzymes— trypsin , elastase , and chymotrypsin —act on the peptides reducing them to smaller peptides.
Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases those that break down peptides. Specifically, carboxypeptidase , dipeptidase , and aminopeptidase play important roles in reducing the peptides to free amino acids.
The amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are summarized in Figure 2 and Table 2. Figure 2. Protein digestion is a multistep process that begins in the stomach and continues through the intestines. Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder.
Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are more widely distributed in the chyme rather than forming large aggregates.
Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.
Why is emulsification important for digestion of lipids? Most complex plant polysaccharides are not digested by humans and enter the colon as a potential food source for the microbiota.
But, in the late s, the extent to which gut bacteria could metabolize this dietary fibre was largely unknown. To bridge this gap, Abigail Salyers and colleagues tested the ability of a wide range of anaerobic bacterial species resident in the human colon to ferment plant polysaccharides as well as intestinal mucins glycosylated proteins that line the gut epithelium.
They found that the bacterial strains had a diverse and inducible ability to break down different substrates, with the largest variety of polysaccharides fermented by Bifidobacterium and Bacteroides species. The researchers proposed that by altering availability of preferred bacterial food sources in the host diet — such as limiting fibre intake — could trigger induction of enzymes capable of degrading the intestinal mucin layer, affecting human health and even colon cancer.
By colonizing the gut of germ-free mice with Bacteroides thetaiotaomicron — which Salyers and colleagues had identified as a human symbiont capable of fermenting a wide range of glycan substrates — the researchers tested, in a physiologically relevant setting, the effects of different diets on expression of bacterial genes.
In mice fed a fibre-rich diet, B. By contrast, in mice fed a diet devoid of complex polysaccharides, the most highly upregulated bacterial genes were those involved in host glycan degradation. The work confirmed in a vertebrate the differential expression of bacterial enzymes dependent on food source, exposing a flexible network of genes for harvesting glycans based on their availability in the host.
Subsequent work on B. Such findings, as well as later studies, underscore the important interplay of host diet and glycan metabolism for gut colonization of human commensals and their persistence in populations over time.
We now know that gut microorganisms harbour thousands of genes involved in catabolism of carbohydrates, but how this enzymatic breadth was acquired remains unclear. One potential source of genetic diversity is horizontal transfer of genes from environmental microorganisms to gut bacteria. In a search for enzymes expressed by a marine Bacteroidetes that degrade sulfated polysaccharides found in edible seaweed such as nori , porphyranases were identified that had a homologue in a gut bacterium, Bacteroides plebeius.
Additional porphyranases and agarases were subsequently identified in the microbiomes of Japanese but not North American individuals. The findings suggest that transfer of genes from marine bacteria on nori was the likely origin of enzymes in the human gut that diversify the ability of bacteria to harvest energy from food sources, such as algal polysaccharides in seaweed, a major dietary component in Japan. Absorption of glucose entails transport from the intestinal lumen, across the epithelium and into blood.
The transporter that carries glucose and galactose into the enterocyte is the sodium-dependent hexose transporter, known more formally as SGLUT As the name indicates, this molecule transports both glucose and sodium ion into the cell and in fact, will not transport either alone. The essence of transport by the sodium-dependent hexose transporter involves a series of conformational changes induced by binding and release of sodium and glucose, and can be summarized as follows:.
Fructose is not co-transported with sodium. Rather it enters the enterocyte by another hexose transporter GLUT5. Once inside the enterocyte, glucose and sodium must be exported from the cell into blood.
But exactly how the bacteria use these protein complexes and sensors in their daily life has been unclear. Now, a new study by Eric Martens, David Bolam, and colleagues has looked into how a pair of the most common species of gut bacteria metabolize polysaccharides, showing that each bacterium is highly specialized. Using a high-throughput system for feeding the bacteria dozens of kinds of carbohydrates, one at a time, and tracking the bacteria's gene expression, they were able to see how these microbes have tailored themselves to fill specific niches in the gut.
These two bacterial species do have a fair amount of overlap in what they are able to subsist on, including the ability to eat more complex carbohydrates than had been thought possible—with PULs encoding enzymes that break down even rhamnogalacturonan II, a very complex carbohydrate that's a common part of plant cell walls, and that had been thought to be resistant to microbes.
But the two species—both from Bacteroides , one of the main genera of gut bacteria in mammals—have diverged to tackle particular kinds of foods.
Bacteroides ovatus can break down hemicelluloses. Bacteroides thetaiotaomicron , though, can't eat these hemicelluloses—but it does have a suite of PULs that help it break down many pectins, as well as other PULs that target complex carbohydrates that human intestinal cells secrete as mucus.
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