Lite coolt om Rubisco

[SIZE=3][FONT=Times New Roman]Rubisco, ribulosbifosfatcarboxylas, är världens vanligaste [/FONT][/SIZE][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]enzym[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3] och ett av de äldsta. Det fanns för mer än tre miljarder år sedan. Större delen av allt liv på [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]jorden[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3] är direkt beroende av solenergi genom den viktiga [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]fotosyntesen[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3] och rubisco som gör om solenergin till kemisk energi som används av [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]organismer[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3] som [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]energikälla[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3]. Man har forskat om rubisco för att hitta ett mer effektivt sätt att använda den eftersom det finns två stora problem med detta enzym. [/SIZE][/FONT]



[FONT=Times New Roman][SIZE=3]För det första så använder rubisco både [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]syrgas[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3] och [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]koldioxid[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3] som [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]substrat[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3], det förstnämnda ämnet leder till [/SIZE][/FONT][COLOR=#cc2200][FONT=Times New Roman][SIZE=3]fotorespiration[/SIZE][/FONT][/COLOR][FONT=Times New Roman][SIZE=3] (syrgas förbrukas) och därmed reducerar fotosyntesens (som sker med koldioxid som bränsle) produktivitet. [/SIZE][/FONT]



[FONT=Times New Roman][SIZE=3]Det andra stora problemet med detta enzym är det är långsamt, den producerar endast två till tre [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#0000ff]molekyler[/COLOR][/SIZE][/FONT][FONT=Times New Roman][SIZE=3] per minut jamfört med vanliga enzym som producerar närmare tusen molekyler per minut.[/SIZE][/FONT]

[FONT=Times New Roman]RUBISCO [/FONT]



[FONT=Times New Roman][SIZE=3]The key enzyme in the Calvin Cycle is the one that catalyzes the transformation of the 5-carbon sugar ribulose-5-phosphate and the single-carbon carbon dioxide to two 3-carbon 3-phosphoglycerates. This reaction has a very high delta-G of -12.4 kcal/mol. The enzyme is called ribulose-1-5-biphosphote carboxylase or Rubisco. Rubisco accounts for 16% of the protein content of the chloroplast and is likely the most abundant protein on Earth. Why is this protein so abundant? Is it because it is so crucial to all life to have a source of carbon fixation? Or perhaps it is because the enzyme is very inefficient and has only evolved once, so it has not had to sustain any competition from more effective carbon fixers. The question is an interesting one to ponder. In fact, we will discover in the [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#800080]next section[/COLOR][/SIZE][/FONT][SIZE=3][FONT=Times New Roman] that Rubisco is, in fact, very inefficient, and that a mechanism has evolved to deal with this handicap.[/FONT][/SIZE]

[FONT=Times New Roman]C3 vs C4 plants[/FONT]



[FONT=Times New Roman][SIZE=3]As mentioned previously, [/SIZE][/FONT][FONT=Times New Roman][SIZE=3][COLOR=#800080]Rubisco [/COLOR][/SIZE][/FONT][SIZE=3][FONT=Times New Roman]is the most abundant enzyme on Earth. It is inarguable that this is a very important enzyme to all life, but it is believed that Rubisco is so abundant because of its inefficiencies. [/FONT][/SIZE]

[SIZE=3][FONT=Times New Roman]Rubisco will sometimes recognize oxygen as a substrate instead of carbon dioxide. In that case, the following set of reactions occur: [/FONT][/SIZE]



[SIZE=3][FONT=Times New Roman]As you can see, this result is very detrimental to photosynthesis. Instead of fixing carbon dioxide into a complex sugar, the plant has made extra work for itself in creating phosphoglycolate, a nearly useless compound. This reaction, using oxygen instead of carbon dioxide, directly competes with the regular reaction, at the same site on the enzyme.[/FONT][/SIZE]

[SIZE=3][FONT=Times New Roman]At 25°C, the rate of the carboxylase reaction is four times that of the oxygenase reaction, so the plant is only about 20% inefficient. However, as temperature rises, the balance in the air between oxygen and carbon dioxide changes (due to changing solubility in the ocean), and the carboxylase reaction is less and less dominant. Plants living in warm climates have to overcome this handicap. In addition, plants living in arid climates have to close the pores in their leaves when it is particularly dry, or they will wither. This also gives the effect of creating a closed environment in which, as carbon dioxide gets used up in photosynthesis, the relative concentration of oxygen increases and the oxygenase reaction begins to dominate. [/FONT][/SIZE]

[SIZE=3][FONT=Times New Roman]A solution has evolved to combat this problem. Plants living in the above mentioned difficult conditions have discovered a way to make the carbon dioxide concentration very high in the immediate environment of Rubisco, so that the oxygenase reaction does not get a chance to happen. [/FONT][/SIZE]



[SIZE=3][FONT=Times New Roman]This pathway is called the C4 pathway because it involves a 4 carbon intermediate in the outer cells. The 4-carbon intermediate brings a molecule of carbon dioxide right into the bundle sheath cells, where it is dropped right next to the location of the Calvin Cycle. In this way, the plant ensures that the concentration of carbon dioxide at the site of Rubisco is very high, so that only the carboxylase and not the oxygenase reaction can take place. The conventional pathway is called the C3 pathway because it involves only the 3-carbon sugars. Note that the C4 pathway still uses the conventional Calvin Cycle with its 3-carbon sugar intermediates; it makes use of 4-carbon sugars to bring the carbon dioxide closer to the site of fixation. [/FONT][/SIZE]

[SIZE=3][FONT=Times New Roman]So why don't all plants adopt the C4 process? Or, more correctly, why don't the C4 plants out-compete the C3 plants, which are inefficient? Well, notice that it takes ATP to bring the carbon dioxide to the Rubisco. In moderate temperatures, the energy burden that this puts on the plant outweighs the advantage of eliminating the one in five times that Rubisco binds oxygen instead of carbon dioxide. In warmer climates, however, the C4 plants win with their novel strategy.[/FONT][/SIZE]