Protein là được chuyển qua liên tục, và chế độ ăn protein bị chuyển hóa thành axit amin. Vì không có cửa hàng cho các axit amin dư thừa, họ sẽ bị chuyển hóa liên tục. Nhóm α-amino (NH3) được lấy ra đầu tiên trong quá trình transamination, và bộ xương carbon kết quả là hoặc chuyển đổi trung gian trao đổi chất có thể được | 424 PDQ PHYSIOLOGY Amino Acid Metabolism Proteins are being turned over continuously and dietary proteins are degraded to amino acids. Since there is no store for excess amino acids they must be degraded constantly. The a-amino group NH3 is removed first in the process of transamination and the resulting carbon skeleton is either converted to metabolic intermediates that can be converted to glucose or oxidized in the Krebs cycle and used to support the energy needs of the body. The carbon skeletons of the 20 standard amino acids are funneled into only seven molecules acetyl CoA acetoacetyl CoA a-ketoglutarate succinyl CoA fumarate oxaloacetate and pyruvate. Amino acids that are degraded to one of the Krebs cycle intermediates a-ketoglutarate succinyl CoA fumarate oxaloacetate or to pyruvate are called glucogenic because they can be converted to phosphoenolpyruvate and then be used for the net synthesis of glucose see Figure 10-4 . Amino acids that are degraded to acetyl CoA or acetoacetyl CoA are called ketogenic because they give rise to ketone bodies. Regulation of Energy Metabolism Long-term control of metabolism is significantly influenced by the glucocorticoid cortisol and the thyroid hormones. Cortisol modulates genes so as to permit hormones such as insulin or glucagon to activate metabolic genes. The thyroid hormones T3 and T4 regulate the level of important metabolic enzymes thereby increasing the capacity for 1 O2 uptake in the liver muscle and adipose tissue 2 gluconeogenesis and glycolysis in the liver 3 lipolysis in adipose tissue and 4 protein breakdown in muscle. In the short and intermediate time frames metabolism is regulated 1 at the cellular level by mechanisms that control the plasma concentration and cell membrane transport of energy substrates and 2 at the whole body level by mechanisms that control body weight. Regulation at the Cellular Level Cellular mechanisms of regulation are driven by plasma glucose concentration and the ratio of plasma
