Function of Urease in Plants with Reference to Legumes: A Review

*Peter S. Joseph, Dickson A. Musa, Evans C. Egwim and A. Uthman*

#### **Abstract**

Urease (urea amidohydrolase, EC 3.5.1.5) is a nickel-containing enzyme produced by plants, fungi, and bacteria that catalyzes the hydrolysis of urea into ammonia and carbamate. Plant (especially legumes) ureases hold a special place in science history, participating on some important landmarks of biochemistry as it was the first enzyme ever to be crystallized in 1926. Finding nickel in urease's active site in 1975 was the first indication of a biological role for this metal. Despite the abundance of urease in tissues and seeds of some members of Legumes families, and its ubiquity in virtually all plants little has been revealed of the roles of urease. This review will explore many faces of these ureases from legumes and other plants, their roles, nutritional relationship between plants and the commensal bacteria with which they associate. In addition, we will explore the possibility that bacteria participate in turnover of the "plant" urea pool. Plant ureases possess insecticidal and fungitoxic properties independent of its ureolytic activity. Altogether, with this review we wanted to invite the readers to take a second look at ureases from versatile plants especially legumes for various biotechnological applications.

**Keywords:** legumes, plant, urease, urea

#### **1. Introduction**

Ureases (urea amidohydrolase, EC 3.5.1.5) are ubiquitous enzymes produced by plants, bacteria and fungi, animals do not produce these metalloenzymes. They are found to be the most proficient known enzymes to date, the enzyme catalyzes the hydrolysis of urea to form carbamate and ammonia; the carbamate then decomposes to form carbon dioxide and another molecule of ammonia, enabling the reaction rate to be faster by at least a factor of 1014 when it is compared to the decomposition of urea by elimination reaction [1–4]. The proficiency of urease Computational modeling brought about a proposal of a value equivalent to 1032 multiplied by the theoretical rate of uncatalyzed hydrolysis of urea [5]. But in solution, it can be debated upon that the value obtained is not visible based on some limitation imposed due to the substrate diffusion in water. Ureases from Plants maintain a special position in the history of science, involving in some relevant events in biochemistry. For example, Urease contributed about three landmarks in the history of Biochemistry. One, the *Canavalia ensiformis* (jack bean) seeds urease isolated and crystallized by a scientist called James B. Sumner, who in 1926 demonstrated the enzymes' proteinaceous nature [6], this findings in 1946 was laureated

with the Nobel Prize in Chemistry. Two, the biological importance of nickel (N2+) was in 1975 recognized as obligatory for urease' catalytic activity after studies conducted by Zerner's and colleagues who reveal the existence of nickel ions in jack bean urease' active site [7]. Three, the identification of a toxin in plant as a urease in the year 2001 may be regarded as another breakthrough that has to do with ureases, which brought about the discovery of properties of these enzymes that are noncatalytic [8]. This discovery increased our knowledge on the functions carried out by these proteins, asides their function in metabolism of nitrogen [9]. This urease enzymes belongs to the super family of phosphotriesterases and amidohydrolases, that possesses in their active sight two catalytic Nickel metal(s) with a few reported exceptions [8, 10]. Takeuchi's findings as stated in Real-Guerra et al. make all this possible after he observed that *Glycine max* (soybean) seeds' crude extracts shows high amounts of urease activity [11]. As at that time, research on urease only focus on microorganisms and in algae. Takeuchi's discovery was the first report that shows the existence of ureases in higher plants. The advantage of this finding is that it gave researchers knowledge on the large availability of urease globally, thereafter, so many other researches on ureases were carried out, taking advantage of the leguminous plant urease having the aim to understand functions of the enzyme. Urease from *G. max* was among the main focus on enzymology development, involving several researches associated with this enzyme which brought about hypothesis that were important to Michaelis and Menten's observation on the reaction rate of enzymes and the substrates they catalyzed [12]. Till date, after those first experiments more than a century ago, urease from legumes continued to generate attention by researchers globally, in various fields such as biochemistry, physiology and genetic.

A leguminous plant belongs to the family Leguminosae otherwise known as Fabaceae. They produce their seeds around a pod [13, 14]. This plant family is large with more than 18,000 species shrubs, climbers, trees and herbs whereby only few has been studied for urease extraction and utilization. Common legumes that have been used for the extraction of urease include mung bean, *Pisumsativum* seeds, peas, some beans species, peanuts, lentils, soybeans, upins, lotus, green beans and sprouts are known as food or grain legumes. Different legumes are shown in **Figure 1**.

**Figure 1.** *Some species of legumes.*

In this review, we shall focus on ureases from legumes, providing information generated over time and exposes some areas that need to be focused by researchers.
