NİŞASTA: BİYOSENTEZİ, GRANÜL YAPISI VE GENETİK MODİFİKASYONLAR

Nişasta bitkilerde fotosentezin temel ürünüdür ve polisakkaritlerin hem fotosentetik hem de fotosentetik olmayan dokulardaki en yaygın depo şeklidir. Nişasta insan ve hayvan beslenmesinde temel olarak tüketilen bir gıda olmakla sınırlı kalmayıp yiyecek endüstrisi başta olmak üzere kağıt, tekstil ve diğer birçok endüstriyel alan için de temel bir ham maddedir. Nişastanın granül yapısı ve şekli, amiloz ve amilopektinin moleküler yapısı, amiloz-amilopektin oranı, lipid, protein ve fosfat miktarı gibi faktörler nişastanın fonksiyonel özelliklerini dolayısıyla endüstriyel kullanım alanını belirler. Örneğin yiyecek endüstrisinde, tatlılar ve kızartılmış ürünlerde yüksek amiloz içeren nişastalar tercih edilirken, dondurulmuş ürünlerde ise amiloz içermeyen nişastalar tercih edilmektedir. Bu sebeple endüstriyel amaçlı kullanılan nişasta genellikle fiziksel, kimyasal veya enzimatik olarak modifiye edilerek yapısal özellikleri ve bununla bağlantılı olarak fonksiyonel özellikleri değiştirilir. Günümüzde ise fonksiyonelliği geliştirilmiş bu tip nişastalar özellikle mutant ve genetik modifikasyona uğratılmış bitkilerden üretilmektedir. Gerek bitki biyoteknolojisindeki gelişmeler gerekse nişasta biyosentezinde rol oynayan enzimler ve özelliklerinin ortaya konması genetik modifikasyonları mümkün kılmıştır. Bu derlemede nişasta granül yapısı, depo dokularda nişasta biyosentezinde rol alan enzimler ve bunların genetik modifikasyonu üzerinde durulmuş olup bu modifikasyonların potansiyel yararları tartışılmıştır.

Anahtar Kelimeler:

Amiloz, amilopektin, granüle bağlı nişasta sentaz, waxy (mumlu) mutant

STARCH: BIOSYNTHESIS, GRANULE STRUCTURE AND GENETIC MODIFICATIONS

Starch is the main product of photosynthesis and its the most dominant reserve polysaccaride that stored in photosynthetic and non-photosynthetic tissues. Starch is a staple food in human and animal diets, but also a raw material widely used for industrial purposes, such as food, paper and textile. Starch granule structure, amylose and amylopectin moleculer structure, amylose and amylopectin ratio, and also lipid, protein and phosphate content are the main determinants that effect the functional properties of starch, in turn, its industrial application. For example, in food industry high amylose starches are prefered in sweet and fried products, while amylose free starches are used in frozen foods. In fact, starch is generally modified by physical, enzymatic or chemical treatments to alter structural and functional properties for endustrial applications. Today starches with improved functionality has also been produced from mutant and genetically modified plants. Increased knowledge about enzymes that are involved in starch biosynthesis and improvement in plant biotechnology made possible to alter starch composition by genetic modifications. This rewiev focus on the starch granule structure, starch biosynthetic enzymes in storage tissues and their genetic modifications with potantial benefits.

Keywords:

Amylose, amylopectin, granule bound starch synthase, waxy mutant,

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