Tag Archives: biochemistry

Glucose Gets Up In the Club

Rather than go through the motions of profuse apology and promises to write more in future, let’s just say I was kidnapped by anti-science, anti-blog, anti-fun desert pirates and leave it at that? OK? OK.

Let’s say I ate a cookie. After all the business of masticating, saturating and dissolving said cookie, it comes time to absorb its nutrients. The bulk of this occurs in the small intestine, which is not only highly folded, but the inner surface is covered in small, finger-like projected villi), which in turn are covered in even smaller finger-like projections (microvilli). It is through these that I can absorb the various carbs and sugars this digested cookie has to offer. “But how?!” You ask. Well, I can explain! And later you can explain to me how I can hear you over the internet.

Anyway, you have all these nutrients hanging out in the lumen (the open space inside the intestine) of the small intestine that need to get from there to the blood stream. But first they have to get through the epithelial cells of the microvilli. They can’t simply diffuse (see here for a diffusion review), the concentration of nutrients within the epithelial cells is too high to allow diffusion. This is where membrane transport proteins come in handy. These are proteins embedded in the lipid bilayer of the cell that, through various means, transport molecules and ions from one side of the membrane to the other. In the small intestine in particular, there is a class of transporters called sodium-dependent secondary active transporters. These proteins use sodium to power the transport of various molecules such as carbs, sugars and amino acids into the epithelial cells of the microvilli. Although many work in similar ways, I’m going to use the sodium-glucose symporter as an example.

First off, let’s have a few definitions so that I don’t leave all y’all in the dust. Secondary active transport involves moving one molecule into a cell against its gradient by coupling it with another molecule or ion that is moving (into or out of the cell) with its gradient. So say you’re trying to get into a club, but the room is too full, so either a friend of yours leaves so that you can get in, or you have an assertive friend with you that drags you in whether there’s room or not. A protein/bouncer that allows that first example would be an antiporter (2+ molecules moved in opposite directions), while the second would be a symporter (2+ molecules moved in the same direction). In the case of the sodium glucose symporter, two sodium ions act as the pushy friend to get glucose into the hopping club that is the epithelial cell. Inside the cell there is a high concentration, but a low (and carefully maintained) concentration of sodium. Sodium’s movement down its gradient actually provides the energy to power glucose transport (or pay glucose’s cover, if you want to keep going with the club analogy).

Image

Yep, that is one hoppin’ club.

 

To get into the blood stream, the process is much simpler. Blood glucose concentration is much lower than that of the epithelial cell, so diffusion is very possible. HOWEVER, glucose still needs help. Dissolved in the cytosol of the epithelial cell, it is hydrated; surrounded by a little cage of water molecules that would need to be stripped on in order for the glucose to pass through the hydrophobic lipid bilayer. Not a terribly favorable reaction. As a result, GLUT-2 transport proteins act to facilitate diffusion into the blood stream. They provide a hyodrophilic environment for the glucose to pass through. Many membrane transporters act as catlysts in this way, lowering the “activation energy” of transport for molecule and ions (much like enzymes catalyzing reactions).

Now that the glucose is successfully transported, there’s just a little housekeeping left. A sodium-potassium ATPase antiporter brings in two potassium ions and boots out three sodium ions. Unlike the sodium-glucose symporter, this is an ATP powered pump. Elsewhere, potassium channels allow for diffusion of potassium ions into and out of the cell to maintain concentration gradients.

Tune in next time and maybe I will explain how blood glucose levels are regulated.

But only if you’re all very good.

So watch it.

 

Source

Sholders, Aaron. “Types of Membrane Transport”. Principles of Biochemistry. Great River Technologies. 2013. eBook.

Sholders, Aaron. “Membrane Transport”. Biochemistry. Colorado State University. Fort Collins, CO. Jan 2013. Lecture

 

Photo credit: Sholders, Aaron. “Types of Membrane Transport”. Principles of Biochemistry. Great River Technologies. 2013. eBook.

Story Time: The Little Protein That Could

Story Time, kiddies! Gather round and I shall tell you a tale of The Little Protein That Could (and Would, Whether you Liked It Or Not).

Once upon a time there was a little protein. It was brand new and only 200 amino acids long. Which I suppose isn’t so little, polypeptide chains can be between 50 and 2000 amino acids long, but it’s certainly small compared to a lot of things., like a tissue or an organ or a schnauzer. Anyway, this was a denatured little protein, just a randomly coiled string of amino acids with only a primary structure to call its own. No secondary or tertiary, let alone quartenary structure! But it dreamed of folding into a functional protein, of finding its native state. Its friends had started out the same way, unfolded and unfunctional, but they had folded and moved on in the world. The little protein could not expect help from them. It couldn’t just try all the conformations available; that would take too long (to the tune of many thousands of years). So the little protein had to find help.

There were scientists studying all the proteins, but they were too distracted by the unpredictable coils of the denatured little protein (they understand very little of such things). Also proteins aren’t sentient and can’t talk. So the little protein had to rely on its self. Looking deep within (its primary structure), the little protein realized it had all the information it needed to folded within its primary structure! Insofar as proteins realize things (which is to say that they don’t). The very amino acids composing it could interact with one another via hydrogen bonds, disulfide bonds and other electrostatic interactions to find its energetically favorable native state.

Once folded, the little protein felt (“felt”) such relief (“relief”); the native state was a lower energy state than when it was unfolded. Now after its long (“long”, a fraction of a second) journey, it could go out into the world and follow its destiny (biological function).

The End.

 

Moral of the story: Look inside yourself and let your amino acids guide you.

 

Source

Sholders, Aaron. “Thermodynamics.” Biochemistry. Colorado State University. Fort Collins, CO. Jan 2013. Lecture

Protein Bangles

Get used to seeing lots of biochemistry in the coming months because guess what class I am taking! Ten points if you said biochemistry, twenty consolation points if you said Intermediate Snare Craft. To start with, y’all need some of the basic facts on protein. And I don’t mean that it’s one of the basic food groups.

First thing’s first: proteins are made up of amino acids, which in turn are composed of codons. I can only define all these things in a very round way, so bear with me. Remember the base pairs of DNA? They connect to two helices and are abbreviated with A, T, C and G. Identical base pairs are found in RNA (With the exception of T being replaced by U), which is like the paternal twin of DNA; they look very similar but are in fact very distinct. RNA provides the instructions for translation, literally the translation of genetic instructions into (hopefully) functioning proteins. Three bases make up a codon, which, depending on the codon, indicate particular amino acids for the translation proteins to assemble into the protein. Because I don’t want to take an even bigger detour, think of RNA as beading instructions. The codons represent an amino acid bead that needs to added to a string until you have the whole bracelet, or whatever.

Now the analogy gets a little funky. Proteins are made up of a combination of twenty beads (nine of which we can’t manufacture and need to consume, hence the nine essential amino acids) which start out as a string (or a peptide chain), but do not stay that way. The strand of beads is only the primary structure. Depending on the composition of the protein, the strand may fold into an alpha helix or a beta pleated sheet. Those are examples of secondary structure. The protein can then fold further into a tertiary structure, and then bind with other, separate strand in an oligomer (protein made of multiple subunit proteins). This oligomer is its quartenary structure. Consider an elaborate beaded necklace, like…this one:

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We have two main strands of beads, with differently shaped groups attached. Now, some necklaces may need fewer separate strands and some need more, just as with proteins. An entire functioning protein may be fibrous and composed of many strands (silk, keratin, collagen, etc) and then it really looks like a necklace. But then there are proteins that are more like the beaded lizards you made in grade school. Bits and bobs EVERYWHERE. Make one out of beaded wire and smoosh it and then you have a globular protein. They’re kind of a mess, but they’re a necessary mess. Think enzymes and hemoglobin.

Homework for tonight: make beaded lizards and/or protein.

Send pictures.

 

Sources

Jackson, Mark. 2009. “Biochemistry Chemical Concepts.” Quick Study Academic Outlines. BarCharts Inc.

Photo credit, Swift’s Jewelry: http://swiftsjewelry.com/swifts_panamshoot23/