Biochemistry. fifth version.
23.1.1. The Digestion and Absorption of Dietary Proteins
Protein digestion begins within the abdomen, the place the acidic setting favors protein denaturation. Denatured proteins are extra accessible as substrates for proteolysis than are native proteins. The first proteolytic enzyme of the abdomen is pepsin, a nonspecific protease that, remarkably, is maximally energetic at pH 2. Thus, pepsin will be energetic within the extremely acidic setting of the abdomen, regardless that different proteins endure denaturation there.
Protein degradation continues within the lumen of the gut owing to the exercise of proteolytic enzymes secreted by the pancreas. These proteins, launched in Chapters 9 and 10 (Sections 9.1 and 10.5), are secreted as inactive zymogens after which transformed into energetic enzymes. The battery of enzymes shows a big selection of specificity, and so the substrates are degraded into free amino acids in addition to di- and tripeptides. Digestion is additional enhanced by proteases, reminiscent of aminopeptidase N, which are positioned within the plasma membrane of the intestinal cells. Aminopeptidases digest proteins from the amino-terminal finish. Single amino acids, in addition to di- and tripeptides, are transported into the intestinal cells from the lumen and subsequently launched into the blood for absorption by different tissues (Determine 23.1).
23.1.2. Mobile Proteins Are Degraded at Totally different Charges
Protein turnover—the degradation and resynthesis of proteins—takes place continually in cells. Though some proteins are very steady, many proteins are brief lived, notably these which are vital in metabolic regulation. Altering the quantities of those proteins can quickly change metabolic patterns. As well as, cells have mechanisms for detecting and eradicating broken proteins. A big proportion of newly synthesized protein molecules are faulty due to errors in translation. Even proteins which are regular when first synthesized could endure oxidative injury or be altered in different methods with the passage of time.
The half-lives of proteins vary over a number of orders of magnitude (Desk 23.1). Ornithine decarboxylase, at roughly 11 minutes, has one of many shortest half-lives of any mammalian protein. This enzyme participates within the synthesis of polyamines, that are mobile cations important for development and differentiation. The lifetime of hemoglobin, alternatively, is proscribed solely by the lifetime of the pink blood cell, and the lens protein, crystallin, by the lifetime of the organism.
– “where does protein digestion begin”
“where does protein digestion begin”