Cleavage of the N-terminal propeptide from the pro-alpha1(I) chain of type I procollagen occurs in the cisternae of the rough endoplasmic reticulum and is mediated by the enzyme prolyl 3-hydroxylase. Once in the Golgi apparatus, the procollagen molecule then undergoes further modifications. First, galactose and glucose residues are added to specific hydroxylysine residues on the pro-alpha chains by galactosyltransferase and glucosyltransferase, respectively. This process is known as glycosylation and is essential for the stability and function of the collagen molecule. Next, the procollagen molecule is packaged into secretory vesicles and transported to the cell surface. At the cell surface, the N- and C-terminal propeptides are cleaved off by specific proteases, resulting in the formation of tropocollagen. Tropocollagen then self-assembles into collagen fibrils, which are the basic structural units of connective tissues. Finally, the collagen fibrils are cross-linked by covalent bonds formed between lysine and hydroxylysine residues, further strengthening the structure of the connective tissues. I型前胶原原α1(I)链N端前肽的切割发生在粗面内质网的潴泡中，由脯氨酰3-羟化酶介导。一旦进入高尔基体，前胶原分子就会经历进一步的修饰。首先，半乳糖和葡萄糖残基分别通过半乳糖基转移酶和葡萄糖基转移酶添加到原α链上特定的羟赖氨酸残基上。这个过程称为糖基化，对胶原分子的稳定性和功能至关重要。接下来，前胶原分子被包装成分泌小泡并运输到细胞表面。在细胞表面，N端和C端前肽被特定的蛋白酶切割掉，形成原胶原。原胶原然后自我组装成胶原纤维，这是结缔组织的基本结构单位。最后，胶原纤维通过赖氨酸和羟赖氨酸残基之间形成的共价键交联，进一步加强结缔组织的结构。
An atypical haem in the cytochrome b(6)f complex.
作者信息
Stroebel David, Choquet Yves, Popot Jean-Luc, Picot Daniel
机构信息
Laboratoire de Physico-Chimie Moléculaire des Membranes Biologiques, CNRS/Université Paris 7, UMR 7099, France.
出版信息
Nature. 2003 Nov 27;426(6965):413-8. doi: 10.1038/nature02155.
Photosystems I and II (PSI and II) are reaction centres that capture light energy in order to drive oxygenic photosynthesis; however, they can only do so by interacting with the multisubunit cytochrome b(6)f complex. This complex receives electrons from PSII and passes them to PSI, pumping protons across the membrane and powering the Q-cycle. Unlike the mitochondrial and bacterial homologue cytochrome bc(1), cytochrome b(6)f can switch to a cyclic mode of electron transfer around PSI using an unknown pathway. Here we present the X-ray structure at 3.1 A of cytochrome b(6)f from the alga Chlamydomonas reinhardtii. The structure bears similarities to cytochrome bc(1) but also exhibits some unique features, such as binding chlorophyll, beta-carotene and an unexpected haem sharing a quinone site. This haem is atypical as it is covalently bound by one thioether linkage and has no axial amino acid ligand. This haem may be the missing link in oxygenic photosynthesis.
光系统I和II（PSI和II）是反应中心，它们捕获光能以驱动光合放氧作用；然而，它们只能通过与多亚基细胞色素b（6）f复合物相互作用来实现这一点。该复合物从PSII接收电子并将其传递给PSI，将质子泵过膜并驱动Q循环。与线粒体和细菌同源物细胞色素bc（1）不同，细胞色素b（6）f可以使用未知途径切换到围绕PSI的循环电子传递模式。在这里，我们展示了莱茵衣藻细胞色素b（6）f在3.1埃分辨率下的X射线结构。该结构与细胞色素bc（1）有相似之处，但也表现出一些独特特征，例如结合叶绿素、β-胡萝卜素以及一个意想不到的与醌位点共享的血红素。这种血红素是非典型的，因为它通过一个硫醚键共价结合且没有轴向氨基酸配体。这种血红素可能是光合放氧作用中缺失的环节。
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